Q.The charge of cathode rays is

Ordinary gases at atmospheric pressure are poor electrical conductors because they contain essentially no free electrons to carry current. However, a specially designed low-pressure apparatus - the gas discharge tube - can be used to make a gas conduct, and it was precisely by studying the conduction that occurs in such a tube that cathode rays, and eventually the electron itself, were discovered.

A typical discharge tube is a long (roughly 50 cm), sealed glass cylinder about 4 cm in diameter, containing a pure gas whose pressure can be reduced using an attached vacuum pump and monitored with a low-pressure gauge. Two metal electrodes - an anode connected to the positive terminal and a cathode connected to the negative terminal of an induction coil (secondary maintained at about 50 kV) - sit at either end. As the pressure is lowered in stages, the tube passes through a striking sequence of visual states: initially no discharge occurs at all; around 100 mm of Hg pressure, irregular crackling streaks of light and sound appear; at around 10 mm of Hg a steady luminous "positive column" fills much of the tube; and as the pressure is reduced further, to around 0.01 mm of Hg, the positive column vanishes, a dark region (Crooke's dark space) appears near the cathode, the glass walls glow green, and invisible "cathode rays" are found streaming from the cathode. …