cathode ray tube experiment kit

Science simulations

Cathode Ray Tube (CRT)

A cathode ray tube (CRT) is a device that produces cathode rays (a stream of electrons).

The inside of the tube is evacuated or filled with a noble gas. When a high voltage is applied to the cathode ray tube, electrons are ejected from the cathode and accelerated toward the anode. Some of the cathode rays can escape through the open hole in the anode. Cathode rays go straight if there is no influence from the environment.

In 1897, British physicist Thomson discovered that this flow was composed of negatively charged particles from the following phenomenon.

• If there is a metallic object in the middle, it will block the flow and create a shadow.
• Cathode rays bend in a magnetic field in the opposite direction to that of the current.
• Cathode rays are attracted to the (+) of the electric field.

Before flat-screen televisions became widespread, fluorescent materials were applied to the end of the cathode ray to emit light, and it was used as a display. (CRT Display)

DIY Cathode Ray Tube: Interacting With Electrons

Introduction: DIY Cathode Ray Tube: Interacting With Electrons

Electrons: they are all around us, but we cannot really see or interact with them. Luckily, there is a device that will let you. A cathode ray tube, or CRT, is a glass tube with electrodes on either end. When there is a vacuum drawn and a high voltage across the two electrodes, a cathode ray forms. This is a stream of electrons that looks like a brilliant blue-purple line of plasma. You can manipulate and bend this stream of electrons with a magnet. This line not only looks cool, but it can be used to prove a scientific theory; electrons are a subatomic unit with a negative charge. I'll explain how it proves this theory later in this instructable. This piece of test equipment that I will show you how to build can be used as an awesome scientific demonstration for any physics and chemistry class, or it can be used just as a way to confirm a long standing scientific principal.

The video below compliments this instructable with a visual demonstration of this Cathode Ray Tube in action.

Lets Get Started!

Step 1: How a CRT Works

CRT's have been around for a long time. In fact, the first one was built in 1897 by scientist Ferdinand Braun. These tubes have been used as television screens for all of the 20th century and for the early part of the 21st century. They are still used widely today as oscilloscope screens, but these cathode ray tubes as screens are a little bit more complicated than the simple cathode ray tube that we are going to be building in this instructable. So many of you may be wondering now, how does a cathode ray tube work?

A cathode ray tube consists of a ray going from the cathode to the anode, and it is made up of electrons. The simplest cathode ray tube is a sealed glass chamber that has electrodes at either end and a port to be able to draw a vacuum from. If you draw a vacuum on the chamber and apply a high voltage to both ends, then a blueish purple glowing line forms. This is the cathode ray. Now the reason this ray forms is because of the high voltage. The cathode, because it has a negative high voltage with respect to the anode, will have a large amount of electrons on it. These electrons want to flow to the anode to equalize the voltage, but cannot because there are too many air molecules blocking their way. If you remove most of the air in the chamber though, then the electrons are able to flow from one electrode to another. Because the vacuum is not perfect, there will still be air molecules in the tube. When the electrons in the ray hit an oxygen atom, it moves to a higher energy state. Because it cannot stay at this high energy state for long, the atom releases this energy as light. This is why the beam has a blueish purple glow.

The reason that you can bend this ray with a magnet is because electrons are negative. This allows them to be manipulated by a magnetic field. It was this property of the cathode ray that let JJ Thompson discover and prove the existence of the electron as a subatomic unit of an atom in 1897. You can read more about his experiment at this website:

http://www.nyu.edu/classes/tuckerman/adv.chem/lect...

These cathode rays are able to be used in televisions because they can be effected by magnetic fields. A TV CRT has an electron gun in the back that shoots a beam of electrons at a phosphor screen. The tube then has two electromagnets that are able to change the electron beam's direction in the X and Y axis'. If you apply two different electronic signals to the electromagnets and varying power to the electron gun, you can create an image on the screen. This is like a paintbrush painting hundreds of horizontal lines of with varying levels of blackness on a canvas.

Well, now that you know how a CRT works and is used today, lets build one!

Step 2: Materials

For this project, you will need a few materials and tools.

For tools, you will need:

• A Bunsen burner (You can use a butane soldering iron, a commercial burner, or you can build your own )
• A screwdriver
• Wire cutters/ strippers
• Hot Glue Gun

For materials, you will need:

• Glass test tubes
• Copper Wire
• Nails/screws(For the electrodes)
• Vinyl aquarium tubing
• alligator clips
• refrigerator compressor
• copper wire
• rubber sheeting
• Mechanical pencil

You will also need a high voltage power supply. This can be almost any power supply that can produce a high voltage at a relatively high current. For this project, I used my ZVS Power supply.

You can read my instructable on how to build it here

Or watch my video on it here:

Step 3: The Vacuum Setup

The vacuum setup is what removes the air from the test tube. My vacuum setup is a wooden vacuum chamber that I built for other experiments, but it can be used for this one too. It functions using a refrigerator compressor connected via a vinyl tube to a wooden platform made of particle board. The tube seals with the copper pipe on compressor and seals to the wooden platform with the tip of a mechanical pencil. The glass chamber of the CRT seals to the wooden base with the vacuum hole with a piece of rubber sheeting with a hole in it. To learn more about this vacuum chamber setup, watch this video:

The vacuum setup is essential to making your CRT work. Without it, there will be too much air for the electrons to freely travel. Now, to making the actual CRT!

Step 4: Blowing a Hole in the Glass

The first step to making a CRT creating a hole for the anode. To do this, first found a glass test-tube. I then set the tube at the end of the fire from my homemade Bunsen burner. This heats the glass to the point of it being pliable. After it is heated, use a pointed object to make an indentation on the tip of the test tube. After this, reheat the test tube on the flame again. Once it is hot, blow into the open end of the tube. This will make a bubble that will eventually pop, creating a hole in the top of the test tube.

Step 5: Fixing the Hole

After you have your hole created, it will be jagged from the popped bubble. to fix this and make it less dangerous, use a butane torch, or your Bunsen burner, to melt the jagged glass and make it more smooth.

Step 6: Adding the Anode

This is probably one of the hardest steps. To add the anode, you will need to take a nail or a screw, and insert it into the hole you made in the previous step. You will need to make sure that the point of the screw is inside the tube and there is a good amount of it hanging out of the tube to connect to your power supply. You then need to heat the glass again, and while holding the nail in the hole, use pliers to crimp the glass around the nail. This step takes a little while, but with patience and a steady hand, it can be done. After you are done with this step, the nail should stay put inside the tube without wiggling or falling out.

Step 7: Hermetically Sealing the Tube

Now, in the previous step, the sealing of the hole will not hermetically seal the tube. By this I mean that despite making sure that the screw does not move at all in the glass, air can still escape. This will make the CRT not work because the air will always get back in. To seal it, you can just use hot glue. To do this, use a hot glue gun to deposit glue around the area where the screw meets the glass. This should seal your tube.

Step 8: Setting Up the Vacuum Chamber and Cathode

The cathode of the CRT will be routed through the vinyl tube from the inside of the CRT to the metal of the fridge compressor vacuum pump. To do this, you will first need to take take a long length of bare copper wire, and push it though the tubing from the compressor end. Stop when you have about an inch of copper wire sticking out of the vacuum port on the CRT end of the vinyl tubing. On the other end, cut the wire to about 3/4 inch and bend it into an arc. Finally, insert the copper wire into the copper pipe of the fridge compressor and the vinyl tubing over the copper pipe. you can now use the metal body of the compressor as the ground, or cathode, of the CRT.

Step 9: Vacuuming Down the CRT

After your tube is made and the cathode/vacuuming rig is set up, turn on the compressor by plugging it in. Then, take a piece of rubber sheeting and cut a small hole in the center. After this, set the rubber on top of the hole and set the CRT on top of the rubber. The glass CRT will suction down to the rubber and air will start being removed. It will take about 2-4 minutes to remove enough air to create a cathode ray.

Step 10: Connecting the Power

While the CRT is being vacuum down, you can connect the ZVS high voltage power supply. This power supply was home built and can provide over 35000 volts at about 10 mA. You will need to use alligator clip wires to connect the cathode, or ground of the power supply to the cathode of the cathode ray tube. This is the metal of the compressor. After the ground is connected, connect the anode(top) of the tube to the anode(red wire) of the power supply. Now, your Cathode Ray Tube setup should be functional.

Step 11: Fire It Up!!

To turn on your CRT, wait until it has a full vacuum, then plug it into a Variac transformer. Turn on the variac, then turn up the voltage until you see a purple-blue glowing line appear in your tube! Now, it is time to play!

Step 12: It Works! Electrons Exist

Your cathode ray tube should now be working. The glow looks fantastic, like some otherworldly streetlight. You can then interact with ray using a hard drive magnet. Just hold the magnet near the tube, and the beam will deflect to either move toward the magnet, or away from it. You are actually able to touch electrons though a magnet! Something you were not able to previously touch is now touchable. It is fun to play with this cathode ray tube with a magnet. You are actually proving that electrons exist and are negatively charged by moving these magnets. Because magnetic fields affect charged particles, the fact that a magnet can effect the ray shows that whatever makes up the rays are negatively charged. In fact, this cathode ray tube is a reenactment of the experiment by J.J. Thompson that proved the existence of the electron as a subatomic particle.

This experiment is fun, but do not run it too long. This tube is dissipating about 100 watts, so without the proper cooling, it will melt the seal and burn anyone who touches it. It also uses lethal high voltage, so DO NOT TOUCH THE ANODE!

Overall, a Cathode Ray Tube is an awesome apparatus to build and really fun to play with. It is also cool because is proves scientific principals.

Thanks for reading and make sure to vote for me in the Untouchable Contest, Power Supply Contest, and Scientific Exploration Contest!

Discovering the electron: JJ Thomson and the Cathode Ray Tube

Concept Introduction: JJ Thomson and the Discovery of the Electron

The discovery of the electron was an important step for physics, chemistry, and all fields of science. JJ Thomson made the discovery using the cathode ray tube. Learn all about the discovery, the importance of the discovery, and JJ Thomson in this tutorial article.

Further Reading on the Electron

Electron Orbital and Electron Shapes Writing Electron Configurations Electron Shells What are valence electrons? Electron Affinity Aufbau Principle

Who was JJ Thomson?

JJ Thomson was an English physicist who is credited with discovery of the electron in 1897. Thompson was born in December 1856 in Manchester, England and was educated at the University of Manchester and then the University of Cambridge, graduating with a degree in mathematics. Thompson made the switch to physics a few years later and began studying the properties of cathode rays. In addition to this work, Thomson also performed the first-ever mass spectrometr y experiments, discovered the first isotope and made important contributions both to the understanding of positively charged particles and electrical conductivity in gases.

Thomson did most of this work while leading the famed Cavendish Laboratory at the University of Cambridge. Although he received the Nobel Prize in physics and not chemistry, Thomson’s contributions to the field of chemistry are numerous. For instance, the discovery of the electron was vital to the development of chemistry today, and it was the first subatomic particle to be discovered. The proton and the neutron would soon follow as the full structure of the atom was discovered.

What is a cathode ray tube and why was it important?

Prior to the discovery of the electron, several scientists suggested that atoms consisted of smaller pieces. Yet until Thomson, no one had determined what these might be. Cathode rays played a critical role in unlocking this mystery. Thomson determined that charged particles much lighter than atoms , particles that we now call electrons made up cathode rays. Cathode rays form when electrons emit from one electrode and travel to another. The transfer occurs due to the application of a voltage in vacuum. Thomson also determined the mass to charge ratio of the electron using a cathode ray tube, another significant discovery.

How did Thomson make these discoveries?

Thomson was able to deflect the cathode ray towards a positively charged plate deduce that the particles in the beam were negatively charged. Then Thomson measured how much various strengths of magnetic fields bent the particles. Using this information Thomson determined the mass to charge ratio of an electron. These were the two critical pieces of information that lead to the discovery of the electron. Thomson was now able to determine that the particles in question were much smaller than atoms, but still highly charged. He finally proved atoms consisted of smaller components, something scientists puzzled over for a long time. Thomson called the particle “corpuscles” , not an electron. George Francis Fitzgerald suggested the name electron.

Why was the discovery of the electron important?

The discovery of the electron was the first step in a long journey towards a better understanding of the atom and chemical bonding. Although Thomson didn’t know it, the electron would turn out to be one of the most important particles in chemistry. We now know the electron forms the basis of all chemical bonds. In turn chemical bonds are essential to the reactions taking place around us every day. Thomson’s work provided the foundation for the work done by many other important scientists such as Einstein, Schrodinger, and Feynman.

Interesting Facts about JJ Thomson

Not only did Thomson receive the Nobel Prize in physics in 1906 , but his son Sir George Paget Thomson won the prize in 1937. A year earlier, in 1936, Thomson wrote an autobiography called “Recollections and Reflections”. He died in 1940, buried near Isaac Newton and Charles Darwin. JJ stands for “Joseph John”. Strangely, another author with the name JJ Thomson wrote a book with the same name in 1975. Thomson had many famous students, including Ernest Rutherford.

Discovery of the Electron: Further Reading

Protons, Neutrons & Electrons Discovering the nucleus with gold foil Millikan oil drop experiment Phase Diagrams

Science in School

Build your own particle accelerator teach article.

Author(s): Andrew Brown, Julian Merkert, Rebecca Wilson

The world’s largest particle accelerator, the LHC, is deepening our understanding of what happened just after the Big Bang. Here’s how to explore the principles of a particle accelerator in your classroom.

When students think of a particle accelerator, they are likely to imagine the world’s largest ‒ CERN’s Large Hadron Collider (LHC). However, not all particle accelerators are used to investigate the origins of the Universe, nor are they in a 27 km circular tunnel that crosses an international border. Much closer to home is the cathode ray tube (CRT) found in old-fashioned computer and television monitors. A CRT is a linear particle accelerator that creates an image on a fluorescent screen by accelerating and deflecting a beam of electrons in a vacuum (figure 1). And although CRTs are many orders of magnitude less powerful than the LHC, the principles of operation are similar ( table 1 ).

Table 1: A comparison of the classroom particle accelerator (the CRT) and CERN’s LHC

Characteristic	CRT	LHC
Pressure (For comparison, a vacuum cleaner has a pressure of 1‒10  atm, and outer space has a pressure of <10 atm)	10 ‒10  atm	10 ‒10  atm
Distance travelled by a particle between collisions	0.1‒100 mm	1‒105 km
Particle type and source	Electrons produced by thermionic emission at the cathode (a heated filament)
Mode of accelerating particles	A potential difference between the anode and cathode	Electronic fields and radio frequencies, synchronised with particle speed
Mode of directing particles	Electrical or magnetic fields	Strong magnetic fields achieved using superconducting electromagnets (4 T in strength)
Mode of focusing particles	Wehnelt cylinder and anode hole	Quadrupole magnets
Ultimate aim	To cause a beam of particles to form an image on a fluorescent screen	To collide the beam of particles with a second beam and observe the result

The activities described below enable students to control the same parameters in a CRT as scientists do at the LHC: creating a particle beam, changing the path of the particles and altering their speed. All four activities could occupy a class for at least half a day, but they could also be used separately in individual lessons. For all activities, the particle accelerator needs to be set up as outlined in the worksheet that can be downloaded w1 .

Producing the free particles

See the list of the necessary materials in the downloadable document w1

• On the CRT power supply unit, disconnect the lead that supplies the voltage to the cathode (see the circuit diagram in the attached worksheet).
• Set the voltage of the auxiliary anode – the anode of the control grid or Wehnelt cylinder – to 10 V.
• Set the voltage of the anode to 30–50 V.
• Set the cathode voltage to 200–300 V.
• Connect the power unit to a source of electricity. Can you see a spot on the fluorescent screen?
• Reconnect the voltage lead to the cathode and repeat the previous step. Now can you see a spot?

About what happens

A spot is only visible on the fluorescent screen when the cathode is connected. The metal filament heats up and its electrons escape in the form of thermionic emission. The high positive potential of the anode relative to the cathode pulls the electrons into a narrow beam that strikes the fluorescent screen, appearing as a spot.

When the power is disconnected and the cathode is not heated, the electrons cannot escape from the surface because their thermal energy is lower than the energy that binds them to the metal nuclei, sometimes called the work function. Consequently, no electron beam is observed and no spot appears on the screen.

How does this compare to the LHC? Instead of electrons, the LHC accelerates beams of protons or lead nuclei (table 1). The protons, however, are produced using a similar technique to the CRT – in this case with an ion source known as a  duoplasmatron . A cathode filament emits electrons into a vacuum chamber containing small amounts of hydrogen gas. The electrons ionise the hydrogen gas, forming a plasma of hydrogen ions (protons) and free electrons. The protons are then confined by magnetic fields and accelerated to become a beam.

Deflecting an electron beam with an electrostatic field

See the list of the necessary materials in the downloadable document w1 .

• On the power supply unit for the deflection plate, alter the voltage first to the left and then to the right deflection plate (between -80 V and +80 V). What happens to the spot on the screen?
• Vary the voltage to the auxiliary anode of the control grid. How does the spot on the screen change?

When the voltage to the left deflection plate is greater than the voltage to the right plate, the spot will be to the left of the screen and vice versa.

This is because an electrostatic field is created when a potential is applied across the plates. The negatively charged electrons are deflected towards the positive plate, which makes them follow a curved path within the electrostatic field.

Once the electrons leave the electrostatic field, they travel in a straight line towards the screen, at the angle at which they left the field. The greater the potential applied to the plate, the wider the deflection angle of the beam.

Increasing the voltage to the control grid brightens and sharpens the spot on the screen because the potential difference between the control grid and the anode is greater than that between the cathode and the anode. The electrons released by the cathode are repulsed by the control grid and focused towards the anode, resulting in a fine electron beam.

Deflecting the beam with magnetism

If you do not have access to a CRT, you could try a comparable demonstration using an old television screen w2 .

• Bring one pole of the bar magnet close to the side of the CRT, beside the path of the beam. What happens to the spot?
• Power up some electromagnetic coils and bring them close to the side of the CRT. What happens to the spot?

When electrons in the beam pass through the magnetic field, they experience a force at right angles both to the direction in which they are travelling and to the magnetic field. This deflects the electron beam. You can work out the direction of force using Fleming’s left hand rule (figure 3).

Electromagnets produce a stronger magnetic field so the beam is deflected more than by the bar magnet.

How does this compare to the LHC? The LHC uses superconducting quadrupole magnets to focus the particle beam. A quadrupole magnet consists of four magnetic poles, positioned so that the field lines cancel each other out at the centre (figure 4). When a particle beam passes through the very centre, where there is no magnetic field, it feels no force. The quadrupole magnet, therefore, pushes the beam into a small cross-section, akin to a lens focusing light. However, each quadrupole magnet only focuses the beam in one direction, so to produce a fully focused beam, successive quadrupole magnets at 90° to each other are used.

Optical lenses can be used as an analogue for quadrupole magnets. Just as a series of quadrupole magnets at 90° to each other focuses the electron beam in the LHC, combining two lenses of the same focal length (one converging/focusing and one diverging/defocusing) results in an overall increased focal length.

The total focal length  F  of a system of two lenses with focal lengths  f 1  and  f 2  separated by the distance  d  is given by:

Because the first lens is focusing and the second defocusing, while their focal length is the same,  f 2  = – f 1 . Substituting this into the formula gives:

The total focal length is increased when two lenses are combined.

Changing the speed of particles

• Alter the voltage of the anode. How does the spot on the screen change?

About what happens?

When the anode voltage is low, there is no electron beam. As the voltage is increased, the spot becomes visible and then brighter.

Increasing the potential difference between the anode and the cathode (by increasing the voltage to the anode) increases the velocity of the electrons towards the screen.

How does this compare to the LHC? The first electrostatic accelerator of the LHC (located inside the proton source) accelerates protons using a potential difference of 90 kV. However, these protons do not reach the velocity that the electrons in the CRT reach with a lower potential. This is due to the higher mass of the protons. Proton accelerators like the LHC, therefore, need more energy to accelerate particles to high speed.

Comprehension questions

• What is the speed of electrons that have been accelerated by a potential difference of 250 V in the CRT?
• What is the speed of protons that have been accelerated by a potential difference of 90 kV at the first electrostatic accelerator of the LHC?

More about CERN

The  European Organization for Nuclear Research (CERN) w3  is one of the world’s most prestigious research centres. Its main mission is fundamental physics – finding out what makes our Universe work, where it came from, and where it is going.

CERN is a member of  EIROforum w4 , the publisher of  Science in School .

Acknowlegement

These activities were developed in Julian Merkert’s thesis during his study at the University of Karlsruhe, Germany, and a two-month stay at CERN. The initial inspiration came from an idea from Prof. Dr Günter Quast at the University of Karlsruhe to “explain particle physics with school experiments”.

Web References

• w1 – Download instructions on how to set up the apparatus in either  Word  or  PDF form .
• w2 – An alternative to activity 3, using an old-fashioned television screen, is described on the website of the  Oxford University physics department  (search for ‘cathode ray tube’) or use the  direct link .
• The  CERN education website  offers resources for schools and information about CERN’s residential courses for teachers
• w4 – Learn more about  EIROforum .
• Hayes E (2012)  Accelerating the pace of science: interview with CERN’s Rolf Heuer .  Science in School   25 : 6-12.
• Landua R (2008)  The LHC: a look inside .  Science in School   10 : 34-45.
• Landua R, Rau M (2008)  The LHC – a step closer to the Big Bang .  Science in School   10 : 26-33.

Institutions

Andrew Brown is a molecular and cellular biology graduate of the University of Bath, UK. After working for Science in School, he returned to the UK and now works for the Royal Institution.

Julian Merkert is a secondary-school physics and mathematics teacher working at St. Dominikus-Gymnasium Karlsruhe, Germany. During his academic studies at the University of Karlsruhe, he produced teaching materials about the LHC at CERN. He has run several teacher programmes, both at CERN and in Germany.

Dr Rebecca Wilson is a planetary scientist working on public and business engagement projects at the Space Research Centre, University of Leicester, UK. She is a project scientist for the UK’s National Space Academy, collaborating with scientists and educators to develop secondary-school revision materials based on space science. She also works for the Space IDEAS Hub, giving small local businesses access to the university’s space-derived expertise

We have all heard about CERN and the particle acceleration experiments conducted there. However, for some it may seem like a place that is very far from the classroom. Despite this physical distance, the project described in this article succeeds in reducing the barrier between the scientists’ working place and the students’ classroom.

The procedure for setting up the apparatus is very detailed, hence making it accessible to teachers. While ensuring that every part of the project is explained in terms of the physics theories involved, the authors have also compared the LHC with the CRT throughout the article. This makes it extremely interesting, apart from being highly instructive.

This article can give rise to a discussion about the work being done at CERN, linked with the origin of the Universe, the progress we have made so far in the exploration of this theory, and the certainties and uncertainties surrounding it!

Catherine Cutajar, Malta

Supporting materials

Instructions on how to set up the apparatus (Word)

Instructions on how to set up the apparatus (Pdf)

Download this article as a PDF

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• Structure of Atom

Cathode Ray Experiment

What is cathode ray tube.

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

Table of Contents

Cathode ray tube, recommended videos.

• J.J.Thomson Experiment

Apparatus Setup

Procedure of the experiment.

• Frequently Asked Questions – FAQs

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

The cathode ray tube (CRT), invented in 1897 by the German physicist Karl Ferdinand Braun, is an evacuated glass envelope containing an electron gun a source of electrons and a fluorescent light, usually with internal or external means to accelerate and redirect the electrons. Light is produced when electrons hit a fluorescent tube.

The electron beam is deflected and modulated in a manner that allows an image to appear on the projector. The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

J. J. Thomson Experiment – The Discovery of Electron

The Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electrons and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

J. J. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes. He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and is called a cathode ray tube for the entire construction.

The experiment Cathode Ray Tube (CRT) conducted by J. J. Thomson, is one of the most well-known physical experiments that led to electron discovery . In addition, the experiment could describe characteristic properties, in essence, its affinity to positive charge, and its charge to mass ratio. This paper describes how J is simulated. J. Thomson experimented with Cathode Ray Tube.

The major contribution of this work is the new approach to modelling this experiment, using the equations of physical laws to describe the electrons’ motion with a great deal of accuracy and precision. The user can manipulate and record the movement of the electrons by assigning various values to the experimental parameters.

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

• Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
• High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.
• The electricity starts flowing as the circuit was complete.
• To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
• The positive pole and negative pole were kept on either side of the discharge ray.
• When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
• This was further confirmed by placing the phosphorescent substance at the end of the discharge ray. It glows when hit by a discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were on the positive side. So the constituents of the discharge tube were negatively charged.

After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom . And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, from all this we learn that the electrons are in fact the basic constituent of all the atoms.

Most of the mass of the atom and all of its positive charge are contained in a small nucleus, called a nucleus. The particle which is positively charged is called a proton. The greater part of an atom’s volume is empty space.

The number of electrons that are dispersed outside the nucleus is the same as the number of positively charged protons in the nucleus. This explains the electrical neutrality of an atom as a whole.

Uses of Cathode Ray Tube

• Used as a most popular television (TV) display.
• X-rays are produced when fast-moving cathode rays are stopped suddenly.
• The screen of a cathode ray oscilloscope, and the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall off the screen pictures are visible on the screen.

Frequently Asked Questions – FAQs

What are cathode ray tubes made of.

The cathode, or the emitter of electrons, is made of a caesium alloy. For many electronic vacuum tube systems, Cesium is used as a cathode, as it releases electrons readily when heated or hit by light.

Where can you find a cathode ray tube?

Cathode rays are streams of electrons observed in vacuum tubes (also called an electron beam or an e-beam). If an evacuated glass tube is fitted with two electrodes and a voltage is applied, it is observed that the glass opposite the negative electrode glows from the electrons emitted from the cathode.

How did JJ Thomson find the electron?

In the year 1897 J.J. Thomson invented the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Thomson realized that the accepted model of an atom did not account for the particles charged negatively or positively.

What are the properties of cathode rays?

They are formed in an evacuated tube via the negative electrode, or cathode, and move toward the anode. They journey straight and cast sharp shadows. They’ve got strength, and they can do the job. Electric and magnetic fields block them, and they have a negative charge.

What do you mean by cathode?

A device’s anode is the terminal on which current flows in from outside. A device’s cathode is the terminal from which current flows out. By present, we mean the traditional positive moment. Because electrons are charged negatively, positive current flowing in is the same as outflowing electrons.

Who discovered the cathode rays?

Studies of cathode-ray began in 1854 when the vacuum tube was improved by Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker. In 1858, Plücker discovered cathode rays by sealing two electrodes inside the tube, evacuating the air and forcing it between the electrode’s electric current.

Which gas is used in the cathode ray experiment?

For better results in a cathode tube experiment, an evacuated (low pressure) tube is filled with hydrogen gas that is the lightest gas (maybe the lightest element) on ionization, giving the maximum charge value to the mass ratio (e / m ratio = 1.76 x 10 ^ 11 coulombs per kg).

What is the Colour of the cathode ray?

Cathode-ray tube (CRT), a vacuum tube which produces images when electron beams strike its phosphorescent surface. CRTs can be monochrome (using one electron gun) or coloured (using usually three electron guns to produce red, green, and blue images that render a multicoloured image when combined).

How cathode rays are formed?

Cathode rays come from the cathode because the cathode is charged negatively. So those rays strike and ionize the gas sample inside the container. The electrons that were ejected from gas ionization travel to the anode. These rays are electrons that are actually produced from the gas ionization inside the tube.

What are cathode rays made of?

Thomson showed that cathode rays were composed of a negatively charged particle, previously unknown, which was later named electron. To render an image on a screen, Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electrical or magnetic fields.

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J.J. Thomson’s Cathode Ray Tube Experiment

In 1897, J.J. Thomson conducted a groundbreaking experiment using a cathode ray tube that revolutionized our understanding of atomic structure and subatomic particles . His experiment, conducted at Cambridge’s Cavendish Laboratory, involved manipulating cathode rays with electric and magnetic fields.

Thomson’s custom-made cathode-ray tubes, created by his skilled glassblower assistant Ebenezer Everett, played a crucial role in the success of his experiments. Through his observations, Thomson identified electrons , the first subatomic particles to be discovered, which were found to be 1,000 times smaller than a hydrogen atom.

This experiment provided evidence that cathode rays were composed of tiny particles, rather than waves in the now-rejected ether. It laid the foundation for our understanding of atomic structure and paved the way for advancements in particle physics . The significance of Thomson’s cathode ray tube experiment continues to resonate in the field of science.

Key Takeaways:

• J.J. Thomson conducted a groundbreaking experiment using a cathode ray tube to study electrons and revolutionize our understanding of atomic structure .
• Thomson’s experiments with the cathode ray tube provided evidence that cathode rays were composed of tiny particles, rather than waves in the now-rejected ether.
• His discovery of electrons and the manipulation of cathode rays laid the foundation for our understanding of atomic structure and subatomic particles .
• The quality of the cathode-ray tubes, as well as the skill of the glassblower, were crucial for the success of Thomson’s experiments.
• Thomson’s experiments and subsequent theories inspired generations of physicists and led to further advancements in particle physics .

The Significance of J.J. Thomson’s Cathode Ray Tube Experiment

J.J. Thomson’s cathode ray tube experiment was a groundbreaking achievement that had a profound impact on our understanding of atomic structure and subatomic particles. His experiment provided evidence for the existence of electrons , the first subatomic particles to be discovered. Thomson’s manipulation of cathode rays and observations of their movement and behavior allowed him to determine the charge-to-mass ratio of electrons.

This experiment led Thomson to propose his “plum pudding” model of the atom, which suggested that atoms consisted of a positively charged “pudding” with negatively charged electrons embedded within it. Thomson’s experiment and subsequent theories about the nature of cathode rays and electrons paved the way for further advancements in particle physics and the development of the modern atomic model.

Thomson’s work inspired future physicists such as Ernest Rutherford and his famous gold foil experiment, which further elucidated the structure of the atom and led to the development of quantum physics. Thomson’s discovery of electrons and his contributions to modern physics solidify his place as one of the pioneers in the field.

“Thomson’s cathode ray tube experiment revolutionized our understanding of atomic structure and subatomic particles. His discovery of the charge-to-mass ratio of electrons laid the foundation for future advancements in the field of particle physics.” – Dr. Emily Johnson, Physics Professor

The Influence on Future Physicists

J.J. Thomson’s cathode ray tube experiment not only advanced our knowledge of atomic structure but also inspired future generations of physicists. His groundbreaking research and discoveries opened up new avenues of exploration within the field of particle physics and shaped the trajectory of scientific advancements.

Thomson’s experiment provided a solid foundation for further studies on the nature of electrons and subatomic particles. The understanding gained from his experiment led to the development of new theories and models that continue to be explored and refined by physicists to this day.

His contributions to the field of particle physics revolutionized our understanding of the microscopic world and set the stage for groundbreaking discoveries in the years to come. Without Thomson’s cathode ray tube experiment, our knowledge of atomic structure and subatomic particles would be vastly different, and the field of particle physics may not have progressed to the extent it has.

Contributions to Modern Physics

J.J. Thomson’s cathode ray tube experiment made significant contributions to modern physics . His discovery of electrons and the understanding of their charge-to-mass ratio laid the groundwork for further advancements in the field.

Thomson’s experiment and subsequent theories about the nature of cathode rays and electrons influenced the development of the modern atomic model, which has been refined and expanded upon over the years. His work paved the way for the development of quantum physics and the exploration of the fundamental building blocks of matter.

Thomson’s legacy as one of the pioneers of modern physics is evident in the continued study of particle physics and the development of new technologies based on his discoveries. His cathode ray tube experiment remains a cornerstone of scientific exploration and a testament to the importance of curiosity and experimentation in advancing our understanding of the universe.

Applications and Legacy of J.J. Thomson’s Cathode Ray Tube Experiment

The cathode ray tube experiment conducted by J.J. Thomson not only revolutionized our understanding of atomic structure and subatomic particles but also had a significant impact beyond the realm of scientific research.

One of the key applications of cathode ray tubes stemming from Thomson’s experiment is in television technology . These cathode ray tubes served as the display screens for early television sets, playing a crucial role in the development of this transformative technology.

Furthermore, cathode ray tubes found their use in oscilloscopes . These devices are essential for visualizing and measuring electrical waveforms, making them invaluable in various scientific and engineering fields.

Thomson’s pioneering work on the discovery of electrons and the development of the cathode ray tube also led to a groundbreaking advancement in medical imaging. By stopping fast-moving cathode rays, X-rays could be produced, enabling medical professionals to visualize internal structures and diagnose conditions accurately.

Recognizing the significance of his contributions, J.J. Thomson was awarded the Nobel Prize in Physics in 1906. This prestigious accolade serves as a testament to the profound impact his experiments had on the field of particle physics and the advancement of scientific knowledge.

Thomson’s cathode ray tube experiment and subsequent discoveries continue to inspire and influence future generations of scientists. His legacy is apparent in the continued study of particle physics and the development of new technologies based on his groundbreaking experiments and theories.

What was J.J. Thomson’s Cathode Ray Tube Experiment?

J.J. Thomson conducted a groundbreaking experiment using a cathode ray tube to study electrons and revolutionize our understanding of atomic structure.

When and where did the experiment take place?

The experiment took place in 1897 at Cambridge’s Cavendish Laboratory, where Thomson spent his scientific career.

How did Thomson manipulate the cathode rays?

Thomson was able to manipulate the cathode rays using electric and magnetic fields.

What did Thomson discover through his experiments?

Thomson was able to identify electrons, the first subatomic particles to be discovered, which were 1,000 times smaller than a hydrogen atom.

What evidence did Thomson’s experiments provide about cathode rays?

Thomson’s experiments provided evidence that cathode rays were composed of tiny particles, rather than waves in the now-rejected ether.

What was the significance of Thomson’s discovery of electrons?

Thomson’s discovery of electrons and the manipulation of cathode rays laid the foundation for our understanding of atomic structure and subatomic particles.

What was Thomson’s proposed model of the atom?

Thomson proposed the “plum pudding” model of the atom, which suggested that atoms consisted of a positively charged “pudding” with negatively charged electrons embedded within it.

What applications did cathode ray tubes have beyond scientific research?

Cathode ray tubes became integral parts of television technology , oscilloscopes , and also revolutionized medical imaging through the production of X-rays.

What was J.J. Thomson’s legacy in the field of particle physics?

Thomson’s work on the discovery of electrons and his profound impact on the field earned him the Nobel Prize in Physics in 1906, inspiring future scientists and advancing our understanding of subatomic particles and atomic structure.

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Cathode Ray Tube (CRT)

Cathode ray tube definition.

A cathode ray tube or CRT is a device that produces cathode rays in a vacuum tube and accelerates them through a magnetic and electric field to strike a fluorescent screen to form images.

Cathode Ray Tube History

The eminent physicist Johann Hittorf discovered cathode rays in 1869 in Crookes tubes. Crookes tubes are partially vacuum tubes having two electrodes kept at a high potential difference to discharge cathode rays from the negatively charged electrode cathode. Arthur Schuster and William Crooks proved that cathode rays are deflected by electric and magnetic fields, respectively. In the year 1897, the English physicist J.J. Thomson’s experiments with cathode rays led to the discovery of the electron , the first subatomic particle to be discovered.

The earliest version of the cathode ray tube, Braun Tube, was invented in 1897 by the German physicist Ferdinand Braun. It employed a cold cathode for working. He used a phosphor-coated mica screen and a diaphragm to produce a visible dot. The cathode beam was deflected by a magnetic field only, in contrast to the discharge tube used earlier in the same year by J.J. Thomson, which employed only electrostatic deflection using two internal plates. Braun is also credited with the invention of the cathode ray tube oscilloscope, also known as Braun’s Electrometer.

In 1907, the cathode ray tube was first used in television when Russian scientist Boris Rosing passed a video signal through it to obtain geometric shapes on the screen. Earlier cathode ray tubes used cold cathodes. However, a hot cathode came into existence after being developed by John B. Johnson and Harry Weiner Weinhart of Western Electric. This type of cathode consists of a thin filament heated to a very high temperature by passing an electric current through it. It uses thermionic emissions in vacuum tubes to release electrons from a target.

The first commercial cathode ray tube television manufacture dates back to 1934 by the company Telefunken in Germany. This curved the path for large-scale manufacture and use of CRT TVs until the recent development of Liquid Crystal Displays, Light Emitting diodes, and Plasma TVs.

Cathode Ray Tube Description

The CRT is composed of three parts.

Electron Gun

This part produces a stream of electrons traveling at very high speeds by the process of thermionic emission. A thin filament is heated up by the passage of alternating current through it. It is used to heat the cathode, generally made of the metal cesium, which releases a stream of electrons when heated to temperatures of about 1750 F. The anode, which is the positively charged electrode, is placed a small distance away and is maintained at a high voltage which forces the cathode rays to gain considerably high accelerations as they move towards it.

The stream of electrons passes through a small aperture in the anode to land in the central part of the tube. There is a grid or a series of grids maintained at a variable potential, which control(s) the intensity of the electron beam reaching the anode. The brightness of the final image formed on the screen is also restricted thus. A monochrome CRT has a single electron gun, whereas a color CRT has three electron guns for the primary colors, red, green, and blue, which overlap among themselves to produce colored images.

Deflection System

The electron stream, after coming out of the anode, tends to spread out in the form of a cone. But it needs to be focused to form a sharp point on the screen. Also, its position on the screen should be as desired. This is achieved by subjecting the beam to magnetic and electric fields perpendicular to each other. The straight path of the beam then gets deflected, and it hits the screen at the desired point. It should be kept in mind that the anode gives it a considerable acceleration of the order of fractions of the speed of light. This endows the beam with very high amounts of energy.

Fluorescent CRT Screen

This part projects the image for the user’s view. It is given a coating of zinc sulfide or phosphorus which can produce fluorescence. When the highly energetic beam of electrons strikes it, its kinetic energy is converted to light energy, thus forming an illuminated spot on the screen. When complex signals are applied to the deflection system, the bright spot races across the screen horizontally and vertically, forming what is called the raster.

The raster scanning takes place in the same way as we would read a book. That is, from left to right, then go down and back to the left and move right to finish reading the line. This continues until the full screen is finished scanning. However, the CRT scan takes place so rapidly every second that the viewer cannot follow the actual movement of the dot but can see the whole image so produced.

Cathode Ray Tube Mechanism Video

Cathode ray tube experiment by j.j.thomson.

It was already known to the scientific fraternity that cathode rays were capable of depositing a charge, thereby proving them to be the carriers of some kind of charge. But they were not really sure whether this charge could be separated from the particles forming the rays. Hence, the celebrated English physicist J. J. Thomson devised an experiment to test the exact nature.

Thomson’s First CRT Experiment

Thomson took a cathode ray tube, and at the place where the electron beam was supposed to strike, he positioned a pair of metal cylinders having slits on them. The pair, in turn, was connected to an electrometer, a device for catching and measuring electric charges. Then, on operating the CRT, in the absence of any electric or magnetic fields, the beam of electrons traveled straight up to the cylinders, passed through the aptly positioned slits, and made the electrometer register a high amount of charge.  So far, the result was quite an expected one.

In the next step, he put a magnet in the vicinity of the cathode ray path that set up a magnetic field. Now, as you may know, an electric field and a magnetic field can never act along the same line. Hence, the charged cathode rays get deflected from their path and give the slits a miss. The electrometer, hence, fails to register anything whatsoever. Thus, he concluded the cathode rays carry the charges along with them wherever they go, and it is impossible to separate the charges from the rays.

Thomson’s Second CRT Experiment

In his second attempt, Thomson tried to deflect the cathode rays by applying an electric field. It could prove the nature of the charge carried by them. There had been attempts before to achieve the end, but they had failed. He thought that if the streams are electrically charged, then they should be deflected by electric fields, but he could not explain why his setup failed to show any such movement.

He later came up with the idea that there was no change from the original path as the stream was covered by a conductor, that is, a layer of ionized air in this case. So he took great pains to make the interior of the tube as close to a vacuum as he could by drawing out all the residual air, and bravo! There was a pronounced deflection in the cathode rays. The great scientist had cleverly put two electrodes, positive and negative, halfway down the tube to produce the electric field. On observing that the beam deflected towards the anode, he could successfully prove that the cathode rays carried one and only one type of charge, negative.

Thomson’s Third CRT Experiment

Thomson tried to calculate the charge-to-mass ratio of the particles constituting the rays and found it to be exceptionally small. That implies the particles have either a very small mass or a very high charge. He decided on the former and gave a bold hypothesis that cathode rays were formed of particles emanating from the atom itself.

Experiment Summary

By using certain modifications in the regular CRT, Thomson’s cathode ray tube experiment proved that cathode rays consist of streams of negatively charged particles having smaller masses than that atoms. It was highly likely for them to be one of the components of atoms.

Cathode Ray Tube Applications

Oscilloscope.

It measures the changes in electrical voltage with time. If the horizontal plate is attached to a voltage source and the vertical to a clocking mechanism, then the variations in the magnitude of the voltage will show up on the CRT monitor in the form of a wave. With an increase in voltage, the line forming the wave shoots up while it comes down if the voltage is low. If, instead of a variable voltage source, the horizontal plates are connected to a circuit, then the arrangement can be used to detect any sudden change in its voltage. Thus, it can be used for troubleshooting purposes.

Televisions

Before the emergence of lightweight LCD and plasma TVs, all televisions were bulky and had cathode ray tubes in them. They had a very fast raster scan rate of about 1/50 th of a second. In a color TV, the persistence of the different colors would last for only the time between two consecutive scans. If it stayed longer, then the tube would produce blurred images. But if the effect of the colors ended before the next scan, then it gave rise to a flickering screen. Modern tube TVs use flat-screen CRTs, unlike their yesteryear counterparts.

Cathode Ray Tube Amusement Device

The predecessor to modern video games, the cathode ray tube amusement device gave the world the first gaming device. The CRT produced electronic signals in the form of a ray of light. Controller knobs in the tube were then used to adjust the trajectories of light so that it could hit on a target imprinted on a clear overlay attached to the CRT display screen. The game was conceptualized on World War II missile displays and created the effect of firing missiles at targets.

Other Applications

Cathode ray tube monitors are widely used as display devices in radars. However, the CRT computer monitor has gradually become obsolete with the introduction of TFT-LCD thin panel monitors.

Health Risks

Ionizing Radiation :  CRTs can emit a small amount of ionizing radiation that needs to be kept under control by the Food and Drug Administration Regulations in 21 C.F.R. 1020.10. However, most CRTs manufactured after 2007 have much lesser emissions than the prescribed limit.

Flicker:  Low refresh rates, 60Hz and below, can produce flicker in most people, although the susceptibility of eyesight to flicker varies from person to person.

Toxicity: Modern-day CRTs may have their rear glass tubes made of leaded glass, which is difficult to dispose of as they can cause an environmental hazard. Some of the older versions also contain cadmium and phosphorus, making the tubes highly toxic. Special cathode ray tube recycling processes fulfilling the norms of the United States Environmental Protection Agency should be followed.

Implosion: Very high levels of vacuum inside a CRT can cause it to implode if there is any damage to the covering glass. This is caused by the high atmospheric pressure, which forces the glass to crack and fly off at high speeds in all directions. Though modern CRTs have strong envelopes to prevent shattering, they should be handled very carefully.

Noise: The signal frequencies used to operate CRTs are of a very high range and are usually imperceptible to the human ear. However, small children can sometimes hear very high-pitched noises near CRT televisions. That is because they have a greater sensitivity to hearing.

The cathode ray tube was a useful invention in Science for the discovery of an important fundamental particle like an electron and also opened up newer arenas of research in atomic Physics. Until about the year 2000, it was the mainstay of televisions all over the world before being forced into oblivion due to the emergence of newer technologies.

https://en.wikipedia.org/wiki/Cathode_ray

https://www.chemteam.info/AtomicStructure/Disc-of-Electron-History.html

https://www.techtarget.com/whatis/definition/cathode-ray-tube-CRT

https://explorable.com/cathode-ray-experiment

http://www.scienceclarified.com/Ca-Ch/Cathode-Ray-Tube.html

Article was last reviewed on Tuesday, May 9, 2023

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I want to ask that in cathode ray tube tv why electrons are never finish which is on cathode while the material have limited electrons

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Cathode Ray Experiments

This topic is part of the HSC Physics course under the section  Structure of The Atom .

HSC Physics Syllabus

investigate, assess and model the experimental evidence supporting the existence and properties of the electron, including:

The discovery of the electron started with observations of various experiments involving cathode ray tubes.

This video discusses the observations and conclusions of a few important cathode ray experiments including:

• use of maltese cross
• use of electric and magnetic fields
• use of paddle wheel

What are Cathode Rays?

Cathode rays were produced in partially evacuated discharged tubes called Crookes tubes (cathode ray tubes).

A simple representation of a gas discharge tube

Similar to a discharge tube, a cathode ray tube consists of two electrodes connected to a high potential difference. The positive electrode is the anode, and the negative electrode is the cathode. The high potential difference causes electrons to move from the anode to the cathode. In a near-vacuum setting where there are few to none air molecules, these electrons can travel unimpeded. 

A simple representation of a cathode ray tube 

A cathode ray was the name given to the observation of these electrons when they were incident on the glass coated with fluorescent material behind the cathode. 

It is important to keep in mind that at the time when cathode rays were observed, scientists were not aware of the existence of electrons. 

Particle vs Wave Nature of Cathodes

Can be deflected by magnetic fields	They are identical, regardless of material used
Can be deflected by electric field	It emanates from the cathode and travels in a straight line
The rays carry energy and momentum	Would cast a shadow of a solid object
Are attracted to positive charges	Could penetrate thin metal foils

Maltese Cross Experiment

• What was done: An anode in the shape of a maltese cross was placed in the path of the cathode ray. 

• Observation:  a shadow of the maltese cross was formed directly behind the anode. 
• Conclusion:  Cathode rays travel in a straight line and can cast a shadow. Some scientists argued that since waves such as light can produce a similar observation, cathode rays are wave in nature. However, this observation could also be produced by particles.  

Cathode Ray Tubes Containing Electric and Magnetic Fields

• What was done:  a metal plate coated with fluorescent material was used to visualise the trajectory of a cathode ray. The cathode ray was passed through a uniform electric field and magnetic field (in separate experiments).

• Observation:  in the presence of an electric field, the path of the cathode ray was deflected towards the positively charged plate. In the presence of a magnetic field, the path of the cathode ray was deflected in a direction that was consisted with a negatively charged mass.
• Conclusion:  cathode rays are streams of negatively charged particles. 

Paddle Wheel Experiment

• What was done:  a glass paddle wheel that could move and rotate freely was placed in the path of the cathode ray.

• Observation: when the glass paddle wheel was struck by the cathode ray, it rotated and moved towards the cathode.
• Conclusion:  cathode rays have momentum and kinetic energy. Therefore, they have mass and are particles in nature.

Are Cathode Rays Waves or Particles?

Thomson's determination of the charge to mass ratio of cathode rays settled the debate of the nature of cathode rays. Thomson demonstrated that cathode rays are particles in nature. 

Previous Section:  Energy Sources of Stars

Next Section:  Thomson's Discovery of The Electron

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The cathode ray tube was a scientific curiosity discovered in the late 19 th century, and a mainstay of display technology in the late 20 th . We now know that the mysterious ‘cathode rays’ are in fact electrons—and we can use magnets to bend their path.

This experiment obviously requires a cathode ray tube filled with gas which glows when electrons hit it. The ideal CRT is enclosed by Helmholtz coils to allow a varying magnetic field to be applied. In the absence of Helmholtz coils, a strong neodymium magnet should suffice to bend the electron beam.

In addition to a cathode ray tube, you’ll probably need a sensitive camera to show your audience the results of this experiment. The beams of electrons are too dim for anything except a very small audience to see directly, and are something of a challenge for video equipment too! A camera with a night mode, or manual control over gain (or ISO) and shutter speed will probably be necessary.

If you’ve not got a cathode ray tube, an old CRT TV or computer monitor and a strong magnet will provide a more qualitative version of this demo.

The demonstrations

• Dim the lights and turn on the camera if you’re using one.
• Turn up the energy of the electron beam until the gas inside the globe is clearly glowing.
• If your CRT doesn’t have Helmholtz coils, simply wave the neodymium magnet near the CRT to show the beam bending. You may need to do this quite slowly if the camera is set to a low frame rate to increase its low light sensitivity.
• If your CRT does have Helmholtz coils, turn up the current in them until the beam bends.
• Having curved the path of the beam, turn up the energy further and show that the curvature decreases with increasing electron energy.
• Apply a higher magnetic field to demonstrate that the curvature can again be increased by increasing the magnetic field strength.

CRT TV/monitor + magnet

• Get an image on the television or computer screen. If it’s a computer screen simply plugging it into a laptop should work. For a TV, many camcorders and digital stills cameras will have an S-video, component or composite connection; older camcorders may have these directly, but newer camcorders or digital cameras may have a bespoke cable which plugs into a mini-USB or similar jack on the camera and feeds out to multiple types of connector for insertion into the TV. A relatively still, bright image or video makes the effect we’re about to observe easier to distinguish.
• Put the strong magnet near to the TV screen. The image will warp, and sweeping trails of colour will appear.
• If the distortion and colours remain after taking the magnet away from the TV, turning it off and on again should force the TV to ‘degauss’ which will fix the problem—this is signified by the distinctive clunk which often accompanies a CRT turning on. Sometimes, often after repeated cycling, the TV will fail to degauss. In this case, turn it off, leave it for a short period, and turn it on again.

Vital statistics

speed of an electron accelerated through 1 V: 600 km/s

strength of the LHC bending magnets: 8.36 T

How it works

The key here is that magnetic fields will bend the path of a moving charged particle, and we can make use of this effect to control a beam. Crucially for the Accelerate! recipe, you need a larger magnetic field to bend a faster-moving particle.

In the cathode ray tube, electrons are ejected from the cathode and accelerated through a voltage, gaining some 600 km/s for every volt they are accelerated through. Some of these fast-moving electrons crash into the gas inside the tube, causing it to glow, which allows us to see the path of the beam. Helmholtz coils can then be used to apply a quantifiable magnetic field by passing a known current through them.

A magnetic field will cause a force to act on the electrons which is perpendicular to both their direction of travel and the magnetic field. This causes a charged particle in a magnetic field to follow a circular path. The faster the motion of the particle, the larger the circle traced out for a given field or, conversely, the larger the field needed for a given radius of curvature of the beam. Making this quantitative point is impossible without control over both particle energy and magnetic field, so this will need to be stated if your demo doesn’t have both of these.

In the case of the CRT TV, the paths of the electrons are distorted by the magnet being brought near the screen. The picture on the screen is dependent on the electrons precisely hitting phosphors on the back of the screen, which emit different colours of light when impacted. The electrons are thus forced to land in the wrong place, causing the distortion of the image and the psychedelic colours.

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Cathode Ray Experiment

The electric experiment by j.j. thomson.

J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly contributed to our understanding of the modern world.

This article is a part of the guide:

• Ben Franklin Kite
• Physics Experiments
• Brownian Movement

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• 1 Physics Experiments
• 2 Ben Franklin Kite
• 3 Brownian Movement
• 4 Cathode Ray Experiment

Like most scientists of that era, he inspired generations of later physicists, from Einstein to Hawking .

His better-known research proved the existence of negatively charged particles, later called electrons, and earned him a deserved Nobel Prize for physics. This research led to further experiments by Bohr and Rutherford, leading to an understanding of the structure of the atom.

What is a Cathode Ray Tube?

Even without consciously realizing it, most of us are already aware of what a cathode ray tube is.

Look at any glowing neon sign or any ‘old-fashioned’ television set, and you are looking at the modern descendants of the cathode ray tube.

Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. This cathode ray also became known as an ‘electron gun’.

Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it.

With more experimentation, researchers found that the ‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be particles.

Physicists knew that the ray carried a negative charge but were not sure whether the charge could be separated from the ray. They debated whether the rays were waves or particles, as they seemed to exhibit some of the properties of both. In response, J. J. Thomson constructed some elegant experiments to find a definitive and comprehensive answer about the nature of cathode rays.

Thomson’s First Cathode Ray Experiment

Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.

His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.

He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.

Thomson's Cathode Ray Second Experiment

Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.

Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.

For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.

As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.

Thomson's Third Experiment

The third experiment was a brilliant piece of scientific deduction and shows how a series of experiments can gradually uncover truths.

Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis .

He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.

Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.

Later Developments

Thomson came up with the initial idea for the structure of the atom, postulating that it consisted of these negatively charged particles swimming in a sea of positive charge. His pupil, Rutherford, developed the idea and came up with the theory that the atom consisted of a positively charged nucleus surrounded by orbiting tiny negative particles, which he called electrons.

Quantum physics has shown things to be a little more complex than this but all quantum physicists owe their legacy to Thomson. Although atoms were known about, as apparently indivisible elementary particles, he was the first to postulate that they had a complicated internal structure.

Thomson's greatest gift to physics was not his experiments, but the next generation of great scientists who studied under him, including Rutherford, Oppenheimer and Aston. These great minds were inspired by him, marking him out as one of the grandfathers of modern physics.

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Martyn Shuttleworth (Sep 22, 2008). Cathode Ray Experiment. Retrieved Sep 07, 2024 from Explorable.com: https://explorable.com/cathode-ray-experiment

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JJ Thompson’s Discovery of Electron: Cathode Ray Tube Experiment Explained

JJ Thomson discovered the electron in 1897 and there are tons of videos about it.  However, most videos miss what JJ Thomson himself said was the motivating factor: a debate about how cathode rays move.  Want to know not only how but why electrons were discovered?

Table of Contents

The start of jj thomson, how thomson discovered electrons: trials and errors, thomson’s conclusion.

A short history of Thomson: Joseph John Thomson, JJ on papers, to friends, and even to his own son [1] , was born in Lancashire, England to a middle class bookseller.  When he was 14 years old, Thomson planned to get an apprenticeship to a locomotive engineer but it had a long waiting list, so, he applied to and was accepted at that very young age to Owen’s college. 

Thompson later recalled that, “the authorities at Owens College thought my admission was such a scandal – I expect they feared that students would soon be coming in perambulators  – that they passed regulations raising the minimum age for admission, so that such a catastrophe should not happen again.

[2] ”  While in school, his father died, and his family didn’t have enough money for the apprenticeship.  Instead, he relied on scholarships at universities – ironically leading him to much greater fame in academia. In 1884, at the tender age of 28, Thomson applied to be the head of the Cavendish Research Institute. 

He mostly applied as a lark and was as surprised as anyone to actually get the position!  “I felt like a fisherman who…had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land. [3] ”  Suddenly, he had incredible resources, stability and ability to research whatever he wished. 

He ended up having an unerring ability to pinpoint interesting phenomena for himself and for others. In fact, a full eight of his research assistants and his son eventually earned Nobel Prizes, but, of course, like Thomson’s own Nobel Prize, that was in the future.

Why did J. J. Thomson discover the electron in 1897?  Well, according to Thomson: “the discovery of the electron began with an attempt to explain the discrepancy between the behavior of cathode rays under magnetic and electric forces [4] .”  What did he mean by that? 

Well, a cathode ray, or a ray in a vacuum tube that emanates from the negative electrode, can be easily moved with a magnet.  This gave a charismatic English chemist named William Crookes the crazy idea that the cathode ray was made of charged particles in 1879! 

However, 5 years later, a young German scientist named Heinrich Hertz found that he could not get the beam to move with parallel plates, or with an electric field.  Hertz decided that Crookes was wrong, if the cathode ray was made of charged particles then it should be attracted to a positive plate and repulsed from a negative plate. 

Ergo, it couldn’t be particles, and Hertz decided it was probably some new kind of electromagnetic wave, like a new kind of ultraviolet light.  Further, in 1892, Hertz accidentally discovered that cathode rays could tunnel through thin pieces of metal, which seemed like further proof that Crookes was so very wrong.

Then, in December of 1895, a French physicist named Jean Perrin used a magnet to direct a cathode ray into and out of an electroscope (called a Faraday cylinder) and measured its charge.  Perrin wrote, “the Faraday cylinder became negatively charged when the cathode rays entered it, and only when they entered it; the cathode rays are thus charged with negative electricity .

[5] ”  This is why JJ Thomson was so confused, he felt that Perrin had, “conclusive evidence that the rays carried a charge of negative electricity” except that, “Hertz found that when they were exposed to an electric force they were not deflected at all.”  What was going on?

In 1896, Thomson wondered if there might have been something wrong with Hertz’s experiment with the two plates.  Thomson knew that the cathode ray tubes that they had only work if there is a little air in the tube and the amount of air needed depended on the shape of the terminals.

Thomson wondered if the air affected the results.  Through trial and error, Thomson found he could get a “stronger” beam by shooting it through a positive anode with a hole in it.  With this system he could evacuate the tube to a much higher degree and, if the vacuum was good enough, the cathode ray was moved by electrically charged plates, “just as negatively electrified particles would be.

[6] ” (If you are wondering why the air affected it, the air became ionized in the high electric field and became conductive.  The conductive air then acted like a Faraday cage shielding the beam from the electric field.)

As stated before, Heinrich Hertz also found that cathode rays could travel through thin solids.  How could a particle do that?  Thomson thought that maybe particles could go through a solid if they were moving really, really fast.  But how to determine how fast a ray was moving? 

Thomson made an electromagnetic gauntlet.  First, Thomson put a magnet near the ray to deflect the ray one-way and plates with electric charge to deflect the ray the other way.  He then added or reduced the charge on the plates so that the forces were balanced and the ray went in a straight line. 

He knew that the force from the magnet depended on the charge of the particle, its speed and the magnetic field (given the letter B).  He also knew that the electric force from the plates only depended on the charge of the particle and the Electric field.  Since these forces were balanced, Thomson could determine the speed of the particles from the ratio of the two fields. 

Thomson found speeds as big as 60,000 miles per second or almost one third of the speed of light.  Thomson recalled, “In all cases when the cathode rays are produced their velocity is much greater than the velocity of any other moving body with which we are acquainted. [7] ”  

Thomson then did something even more ingenious; he removed the magnetic field.  Now, he had a beam of particles moving at a known speed with a single force on them.  They would fall, as Thomson said, “like a bullet projected horizontally with a velocity v and falling under gravity [8] ”.  

Note that these “bullets” are falling because of the force between their charge and the charges on the electric plates as gravity is too small on such light objects to be influential.  By measuring the distance the bullets went he could determine the time they were in the tube and by the distance they “fell” Thomson could determine their acceleration. 

Using F=ma Thomson determine the ratio of the charge on the particle to the mass (or e/m).  He found some very interesting results.  First, no matter what variables he changed in the experiment, the value of e/m was constant.  “We may… use any kind of substance we please for the electrodes and fill the tube with gas of any kind and yet the value of e/m will remain the same.

[9] ”  This was a revolutionary result.  Thomson concluded that everything contained these tiny little things that he called corpuscles (and we call electrons).  He also deduced that the “corpuscles” in one item are exactly the same as the “corpuscles” in another.  So, for example, an oxygen molecule contains the same kind of electrons as a piece of gold!  Atoms are the building blocks of matter but inside the atoms (called subatomic) are these tiny electrons that are the same for everything .

The other result he found was that the value of e/m was gigantic, 1,700 times bigger than the value for a charged Hydrogen atom, the object with the largest value of e/m before this experiment.   So, either the “corpuscle” had a ridiculously large charge or it was, well, ridiculously small.   

A student of Thomson’s named C. T. R. Wilson had experimented with slowly falling water droplets that found that the charge on the corpuscles were, to the accuracy of the experiment, the same as the charge on a charged Hydrogen atom!   Thomson concluded that his corpuscles were just very, very, tiny, about 1,700 times smaller then the Hydrogen atom [1] .  These experiments lead Thomson to come to some interesting conclusions:

• Electrons are in everything and are well over a thousand times smaller then even the smallest atom. 
• Benjamin Franklin thought positive objects had too much “electrical fire” and negative had too little.  Really, positive objects have too few electrons and negative have too many.  Oops.
• Although since Franklin, people thought current flowed from the positive side to the negative, really, the electrons are flowing the other way.  When a person talks about “current” that flows from positive to negative they are talking about something that is not real!   True “electric current” flows from negative to positive and is the real way the electrons move. [although by the time that people believed J.J. Thomson, it was too late to change our electronics, so people just decided to stick with “current” going the wrong way!]
• Since electrons are tiny and in everything but most things have a neutral charge, and because solid objects are solid, the electrons must be swimming in a sea or soup of positive charges.  Like raisons in a raison cookie.

The first three are still considered correct over one hundred years later.  The forth theory, the “plum pudding model” named after a truly English “desert” with raisins in sweet bread that the English torture people with during Christmas, was proposed by Thomson in 1904. 

In 1908, a former student of Thomson’snamed Ernest Rutherford was experimenting with radiation, and inadvertently demolished the “plum pudding model” in the process.  However, before I can get into Rutherford’s gold foil experiment, I first want to talk about what was going on in France concurrent to Thomson’s experiments. 

This is a story of how a new mother working mostly in a converted shed discovered and named the radium that Rutherford was experimenting with.  That woman’s name was Marie Sklodowska Curie, and that story is next time on the Lightning Tamers.

[1] the current number is 1,836 but Thomson got pretty close

[1] p 14 “Flash of the Cathode Rays: A History of JJ Thomson’s Electron” Dahl

[2] Thompson, J.J. Recollections and Reflections p. 2 Referred to in Davis & Falconer JJ. Thompson and the Discovery of the Electron 2002 p. 3

[3] Thomson, Joseph John Recollections and Reflections p. 98 quoted in Davis, E.A & Falconer, Isabel JJ Thomson and the Discovery of the Electron 2002 p. 35

[4]   Thomson, JJ Recollections and Reflections p. 332-3

[5] “New Experiments on the Kathode Rays” Jean Perrin, December 30, 1985 translation appeared in Nature, Volume 53, p 298-9, January 30, 1896

[6] Nobel Prize speech?

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Cathode Ray Tube, Straight-in Tube Experiment Equipment, Physics Teaching Equipment, Scientific kit

• This product demonstrates the phenomenon that the cathode ray tube moves in a straight line and can be blocked by metal.
• The product consists of blister, metal baffle, bracket, cathode, anode, bakelite seat, etc.
• Demonstration of straight into the cathode ray tube is best carried out in a dark room.
• During the demonstration, A2 emits cathode rays, and a shadow with the same shape as the metal baffle will appear on the glass wall of the large-end screen of the ray tube. Bright yellow-green fluorescence appeared on the glaze around the shadow.
• It can be explained that the cathode ray moves along a straight line, and its ability to penetrate the metal sheet is very weak.

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Cathode Ray Tube Experiment

The Cathode Ray Tube Experiment stands as a pivotal milestone in the history of physics, heralding a new era of scientific inquiry and technological innovation. Developed in the late 19th century, this experiment, primarily conducted by scientists such as J.J. Thomson, revolutionized our understanding of the nature of electricity and the structure of atoms. By observing the behavior of cathode rays in a vacuum tube, researchers made groundbreaking discoveries about the properties of electrons, laying the foundation for modern electronics and paving the way for advancements in fields ranging from telecommunications to particle physics.

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• How do isotopes affect average atomic mass?
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• Thomson discovered a subatomic particle while working with the cathode ray tube. What is the particle known as?
• If you put hot water in a test tube, pour food coloring in it, and then put it into a beaker with cold water, what happens? And why does that happen?
• Why are isotopes electrically neutral?
• Before the discovery of electrons how were charges thought of?
• What is the colour of cathode rays?
• How does a cathode ray tube in TV work?
• What are some examples of anodes?
• Does potassium metal have isotopes?
• In Thomson's experiment, why was the glowing beam repelled by a negatively charged plate?
• How did J.J. Thomson know the electron was negative?
• Who proved j.j thomson incorrect with his cathode ray tube experiments?
• Please explain me why we use blue cobalt glass in flame test?
• What are electric charges generated by friction?
• Calculate the energy, in joules, of a photon of green light having a wavelength of 562nm?
• Indicate which of the following photons can cause emission of photoelectrons from the surface of gallium ( 6.7 × 10–19 J)? A) 300 nm B) 400 nm C)