Cathode Ray Tube Experiment and Discovery of Electrons

Discovery Of Electron 

In the later part of the nineteenth century, scientists began to experiment with glass tubes in which a high-voltage electric current was passed through a gas at low pressure. Under ordinary conditions, gases are poor conductors of electricity, however when a high voltage is applied to them at low pressure, the gases behave as conductors and current flows through them in the form of rays, which interact with the glass tube and cause the glass tube to glow. This flow of electricity is called an electric discharge, and so the tube is called discharge tube. The existence of these rays was studied first by William Crookes in 1879 using discharge tube, which is a long glass tube, sealed at both the ends and fitted with two metal electrodes. It is also known as Crookes tube. The tubes are connected to a vacuum pump which is used to reduce the pressure of gas in the tube.

 

Tests soon revealed that these rays were, in fact, negatively charged particles moving from the negative electrode (the cathode) to the positive electrode (the anode). It was Benjamin Franklin who decided which electrode was positive and which was negative. These rays were called emissions rays, and because the rays came from the cathode, they were called cathode rays. If a hole is made in the anode, the flow of the current from cathode to anode may be checked. If the tube behind the anode is coated with zinc sulphide, then a bright spot on the coating is developed.

Thomson Experiment – Determination of Mass  to Charge Ratio 

In 1897, the British physicist J.J. Thomson constructed a special cathode ray tube to make quantitative measurements [the ratio of electrical charge (e) to the mass of electron (me)] of the properties of cathode rays. In some ways, the cathode ray tube he used was similar to a TV picture tube. In Thomson’s tube, a beam of cathode rays was focused on a glass surface coated with a phosphor; this surface glows when the cathode rays strike on it. The results of these experiments established the following properties of cathode rays: 

 1. These cathode rays flow from cathode to anode in straight line, unless influenced by electrical or magnetic fields. 

2. These rays are not visible as such and can be detected by fluorescent or phosphorescent materials which glow when these rays fall on them. 

3. That cathode rays are negatively charged. This can be proved since they behave like negatively charged particles under the influence of electric and magnetic fields. 

4. The experiments performed using the cathode ray tube demonstrated that cathode ray particles are in all matter. They are, in fact, electrons. The properties of electrons are independent of electrode material and nature of the gas. 

5. Cathode rays are made up of material particles and this was demonstrated by the fact that they could rotate the light paddlewheel mounted on an axle.  

6. When the cathode rays strike the metal foil, it becomes heated, showing that cathode rays produce heating effect. 

7. They show penetrating effect and ionize the gas through which they pass.  This property is exploited in making television sets containing cathode ray tubes. 

The cathode ray beam passed between the poles of a magnet and between a pair of metal electrodes that could be given electrical charges. When electrical or magnetic fields are applied perpendicular to the path of charged particles, they deviate from their path and the amount of deviation depends on: 

1. The magnitude of charge on the particle. Greater the magnitude of charge, greater is the interaction with the field, and hence higher is the deviation. 

2. The mass of the charged particle. Heavier particles are deviated to a lesser extent. 

3. The strength of the magnetic field or the voltage applied across the path of the charged particles. 

The magnetic field tends to bend the beam in one direction whereas the charged electrodes bend the beam in the opposite direction. By adjusting the charge on the electrodes, the two effects can be made to cancel, and from the amount of charge on the electrodes required to balance the effect of the magnetic field, Thomson was able to calculate the first bit of quantitative information about a cathode ray particle – the ratio of its charge to its mass as 1.758820 × 10^11 C kg−1, where the coulomb (C) is a standard unit of electrical charge.