Electric Discharge Through Gases

Under normal atmospheric conditions, gases are poor conductors of electricity because they contain essentially no free electrons available to carry current. However, with a special low-pressure arrangement, a gas can be made to conduct - and studying exactly how it does so is what revealed the existence of the electron in the first place.

The apparatus used for this is the gas discharge tube: a long, closed glass tube (roughly 50 cm long, 4 cm in diameter) containing a pure gas at controllable pressure, with a side opening connected to a vacuum pump and a low-pressure gauge, and two metal electrodes (an anode connected to the positive terminal, and a cathode connected to the negative terminal) connected to an induction coil's secondary winding held at about 50 kV.

As the pressure inside the tube is reduced step by step using the vacuum pump, the tube passes through a sequence of distinct visual stages. At around 10 mm of Hg, irregular crackling streaks of light appear. At around 10 mm of Hg (lower still), a steady luminous "positive column" forms stretching from the anode toward the cathode. As the pressure drops further to about 0.01 mm of Hg, this positive column disappears, and a dark region called Crooke's dark space appears near the cathode while the glass walls of the tube begin to glow green. At this stage, invisible rays are streaming from the cathode - these are the cathode rays, later identified as a beam of electrons. …