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.
These cathode rays turned out to be a beam of electrons, and their properties were established experimentally: they travel in straight lines at very high speed (of order 107 m s−1, up to about one-tenth the speed of light), carrying both energy and momentum; they are deflected by electric and magnetic fields in a direction showing they carry negative charge; they heat matter they strike, expose photographic plates and cause fluorescence in certain minerals; they produce X-rays when they strike a heavy-atomic-weight target; and they ionize whatever gas they pass through. It was precisely this deflectable, negatively-charged cathode-ray beam that J.J. Thomson went on to use, in his famous 1897 experiment, to measure the electron's specific charge e/m.