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Cathode Ray Tube Experiment I

February 18, 2013

Today we are aware about the subatomic particles in an atom with their all properties.

But in eighteenth century, the properties like charge, mass etc were a big question with

all the scientists. In 1897 John Joseph (J.J.) Thomson a physics professor at the famous

Cavendish Laboratory at Cambridge University showed that by measuring the bending of a

beam of electrons in an electric field and in a magnetic field, we can determine the charge-

to-mass ratio (q/me) for the particles of cathode rays called as electrons.

Let’s discuss the Cathode ray tube experiments. Start with who conducted the cathode ray

tube experiment. It was J.J. Thomson in 1897. He investigated the cathode rays at high

voltage in an evacuated discharge tube to calculate velocity and deflection of the beam

which later used for calculate the ratio of electric charge to mass of the cathode ray.

In the Thomsons Cathode Ray tube Experiment, he used a one meter long CRT (also

called as electron gun) which was hand-made. It was a long glass tube evacuated and

then sealed. There were two plates about midway in the CRT which were connected to a

powerful electric battery to create a strong electrical field through the way from where the

rays passed. He also could use magnets on either side of the straight portion of the tube

just to the right of the electrical plates. The bending of rays from straight line by using either

the electric field or the magnetic field used to calculate the charge/mass ratio of electrons.

Now a days we are using CRT in glowing neon sign or any ‘old-fashioned’ television sets.
Later the same experiment improved by using certain types of glass which produced a
fluorescent glow at the positive end of the tube. This type of CRT’s discovered by William
Crookes. This type of CRT was coated with a fluorescing material at the positive end. This
end would produce a focused ‘dot’ as the electron gun hit it. You can find much Cathode ray
tube animation on line to understand the concept.

Thomson used magnetic field to prove that the ‘rays’ emitted from the electron gun were

inseparable from the latent charge. He used a metal cylinder on one end of CRT with two
slits and electrometers to measure small electric charges. He found no activity by applying a
magnetic field across the tube and concluded that the negative charge and the ray were
inseparable and intertwined. Second time he proved that the rays carried a negative charge
by using deflection in an electric field.


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