Cathode Ray Experiment | Apparatus, Working

Cathode Rays Experiment

cathode ray experiment

J.J.Thomson conducts three experiments with cathode ray experiment.


In 1897, great physician J.J. Thomson conducted his first cathode ray tube experiment to prove that rays emitted from an electron gun are inseparable from the latent charge.

He built his cathode ray tube with a metal cylinder on the other end. The metal had two small diversions(slits), leading to an electrometer to measure small electric charges.

From the first experiment, he discovered that the electrometers stopped measuring electric charge. He deduced that the electric charge and the cathode rays must be combined and are the same entity.


Then, he conducted a Second experiment to prove the charge carried by the cathode rays was negative or positive. He put a negatively charged metal plate on one side of the cathode rays to go past the anode and a positively charged metal plate on the other side.

Instead of an electrometer at one end of the Cathode Ray Tube, he used a fluorescent-coated tube that would glow were the cathode ray hit it. When the charged metal plates were introduced, he found that the cathode rays bent away from the negative plate and towards the positive plate.

This proved that the cathode rays were negatively charged.


Then, he performed the third experiment to know the particles’ nature and reduce the particle’s mass as they had too small of a mass to be calculated directly.

For the experiment, he used the cathode ray tube with a high applied potential difference between the two electrodes, with the negatively charged cathode produced the cathode rays. He had already deduced that the particles were negatively charged.

Firstly, he applied an electric field in the path between anode and cathode and measured the deflections from the straight path. Now he applied a magnetic field across the cathode ray tube by using an external magnetic field. The magnetic field deflects the cathode ray.

Now he changed the direction of the external magnetic field and found that the beam of electrons is deflected in the opposite direction.

This experiment concluded that the electrostatic deflection is the same as the electromagnetic deflection for the cathode rays. He was able to calculate the charge-to-mass ratio of the electron.

After these three experiments, he deduced that inside the atom, there consists a subatomic particle, named initially ‘corpuscle,’ then changed to ‘electron,’ which is 1800 times lighter than the mass of hydrogen atom (Lightest atom).

Apparatus Setup

cathode ray apparatus setup

The experiment apparatus 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.

Working of Cathode Ray Tubes

working of cathode ray tube experiment

Before we see how J.J. Thomson used the cathode ray tube to discover the electron, we need to know how a cathode ray tube works. As seen in the diagram below, we’ll look at a basic CRT like what J.J. Thomson would’ve used.

The CRT consists of several elements, starting with a vacuum-sealed tube to keep air out of it. On one side of the inside of the tube, there’s a cathode and an anode.

The cathode is a negatively charged conductor, and the anode is a positively charged conductor. Electrons, which have a negative charge, flow off the cathode and are attracted towards the anode. A small hole in the anode allows some electrons to pass through it, creating a beam of electrons. On the opposite side of the tube is a coating that glows when struck by the electrons.

This allowed J. J. Thomson to see where the electron beam was hitting. Before his experiment, we didn’t know electrons existed. So, no one was calling it an electron beam. Instead, what flowed off the cathode toward the anode were called ‘cathode rays.’ Hence the name cathode ray tube.

Procedure of Cathode Ray Experiment

1. Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.

2. High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.

3. The electricity starts flowing as the circuit was complete.

4. 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.

5. The positive pole and negative pole were kept on either side of the discharge ray.

6. When the dipoles were applied, the ray was repelled by the negative pole, and it was deflected towards the positive pole.

7. 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 towards the positive side. So the constituents of the discharge tube were negatively charged.

Conditions for Cathode Ray Experiment

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. 

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