Discharge tubes are devices that produce light as a result of an electrical current passing through a gas, usually at less than normal atmospheric pressure. They are used as energy-efficient lamps for indoor and outdoor lighting.
Naturally occurring luminous discharges include phenomena such as lightning and Saint Elmo’s fire—the glow that extends from steeples and ships’ masts during heavy storms. The first person to harness electrical discharges in evacuated glass tubes was Heinrich Geissler, a German glassblower and inventor. In 1858, Geissler invented a hand pump, which he used to remove air from glass tubes in whose walls electrodes were embedded. Other scientists who studied electrical discharges through low-pressure gases included Faraday, Crookes, and Thomson. The devices built by these workers were the forerunners of cathode-ray tubes, electronic vacuum tubes, and gas-discharge lamps. These early tubes differed in the shape of the tube, the distance between the electrodes, the operating voltage, the nature of the filling gas or vapor, and the pressure in the tube.
Basic principles
For a substance to conduct electricity, it must possess charged particles that are free to move. Under most circumstances, gases are excellent insulators: they consist entirely of electrically neutral particles. Under exceptional conditions, electrons break away from neutral atoms or molecules to leave positively charged ions. This process, called ionization, makes a gas able to conduct an electrical current.
The process of ionization requires an input of energy. Atoms and molecules can acquire the energy for ionization from collisions with cosmic rays or other fast-moving particles or by absorbing high-energy photons, such as gamma rays.
In a discharge tube, ionization is initiated by a strong electric field between the electrodes. Once a few electrons break free, they accelerate toward the positive electrode. The progress of the free electrons is interrupted by collisions with neutral atoms or molecules. If the free electrons have sufficient kinetic energy to cause ionization when they collide, an avalanche of ionizations results, and the gas becomes ionized and conducting.
Whether or not an avalanche of ionizations occurs depends on the ionization energy of the gas, its pressure, and the strength of the electric field. The pressure of the gas determines how far free electrons travel between collisions: at high pressure, free electrons tend to collide before they have enough energy to cause ionization. Raising the strength of the electric field increases the acceleration of free electrons, so the avalanche effect can happen at higher pressures. Note that the strength of the electric field depends on the voltage and distance between electrodes—doubling the gap halves the field.
The passage of current through a gas causes it to become hot. Eventually, the thermal energy of the gas becomes sufficient to cause ionizations through high-energy collisions. In this state, described as plasma, the gas is highly conductive. In a plasma, electrons are continually knocked out of atoms and recaptured, emitting light as they fall into the empty orbitals of ions.
