The Discovery of Cathode Rays
At atmospheric pressure, air is normally an insulator. However,
if two electrodes are placed close together and a high potential
difference applied between them, the air becomes conducting and
a spark jumps across the gap as happens, for example, between
the electrodes of a Wimshurst machine.
If the electrodes are placed in a tube which is evacuated until the pressure has fallen to about 1 mm of mercury, the gas will conduct electricity at a considerably lower potential difference.
Electric discharges through gases at low pressures are not a modern phenomena. As early as 1670, Picard, a French scientist, observed a bluish phosphorescence in the Torricellian vacuum of a barometer, after the mercury column had been shaken up and down.
Lord Charles Cavendish also showed that an electric discharge could be sent through the Torricellian vacuum. Faraday also spent some time (1836) investigating electric discharges through gases.
However, it was not until the latter half of the last century that a determined effort was made to find an explanation for the observed phenomena.
In 1869 Johann Hittorf used an aluminium Maltese cross as the anode in a discharge tube. He found that a sharp shadow of the cross was cast on the end of the tube. This experiment suggested that some sort of radiation was travelling in straight lines from the cathode to the anode. Eugen Goldstein, a German physicist, showed in 1876 that sharp shadows were cast by objects in the discharge tube, even when the cathode had an extended area. He called the radiation kathodenstrahlen or cathode rays.
When a current is passed through a heating element an electrode is heated and this will emit a beam of particles, electrons. These are then passed through a wire mesh which is charged negatively to add control to this flow or beam of electrons. This will produce a fine beam and it determines the intensity or brightness of the electrons which hit the screen.
The beam is then passed through two cylindrical Anodes, positive with respect to the Cathode, will accelerate the electrons to speeds of 107 m/s. Theses Anodes focus the electrons onto a small spot on the screen.
The beam of electrons are then passed through a series of parallel plates, two pairs. These are the X-Plates and the Y-Plates. The Y-Plates alters the vertical position and the X-Plates alters the horizontal position.
The works of Sir William Crookes, an English scientist, and J. J. Thomson (1856-1940), an English physicist,led them to postulate that Cathode Rays have the following properties:
1. They travel in straight lines.
2. They transport energy.
3. They transport a charge.