Cathode Rays

Cathode Rays:

Conduction of electricity through gases has resulted in several important discoveries. In order to study electric discharge in gases at low pressures a discharge tube having electrodes at both of its ends and a side tube in the middle is taken. The side tube can be connected to a Vacuum pump. A very high voltage is applied across the two electrodes.

As the pressure of the gas is gradually reduced, at a certain stage an electric spark is produced with a cracking sound. If the pressure is further reduced, the spark gets transformed into a general glow. Practically the whole space between the electrodes starts glowing with the typical color. If the pressure of the gas is further reduced, the glow diminishes and then practically disappears. However, under these conditions, the walls of the glass tube begin to glow with greenish light. Experimental studies show that the greenish fluorescence of the glass wall is caused by the impact of certain invisible rays coming from the cathode of the discharge tube.

The invisible rays coming from the cathode of the discharge tube are known as cathode rays.

Properties of Cathode Rays:

(1) Travel in a Straight Line- If an obstacle is placed in the path of the cathode rays, the geometrical shadow of the obstacle does not show fluorescence. This experiment also shows that cathode rays are emitted from the cathode.

(2) Emerge Normally from the Cathode- If the cathode is concave, the cathode rays come to a focus. If a piece of metal is kept at the focus, it gets heated to a high temperature.

(3) Exert Mechanical Pressure- If a light wheel capable of rotation on two parallel rails is introduced in the path of cathode rays, the wheel is set in motion.

(4) Penetrate Samll Thickness- Cathode rays cannot be stopped by thin metallic foils.

(5) Excite Fluorescence- When cathode rays are allowed to fall on certain substances like zinc sulfide, the substances glow with typical light. When the cathode ray strikes a hard material, it becomes a source of X-rays.

(6) Produce Ionization- Cathode rays can cause ionization in the gas through which they pass.

(7) Affect Photographic Plate- Cathode rays affect a photographic plate.

(8) Deflected by Magnetic Field- The cathode rays are deflected when a magnet is brought near the discharge tube. The direction of deflection suggests that the cathode rays are constituted by negatively charged particles.

(9) Deflected by Electric Field- Cathode rays are deflected by electric fields. The direction of deflection clearly shows that the cathode rays contain negatively charged particles.

Cathode rays are nothing but streams of charged particles, called electrons. These particles are invisible. Their presence is detected by the fluorescence caused by their impact on the walls coated with zinc sulfide. It was a great challenge to measure the charge (e) and mass (m) of these fast-moving invisible particles. As it was very difficult to measure e and m separately for electrons. J. J. Thomson designed a novel method to find the value of e/m for them. The value of e was later obtained by Millikan.

The most recent and accurate measurement yields the following values for e, m, and e/m for electrons: