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506 RI( HARDSON'S BLUEBOOK OF PROJECTION
as a half-wave rectifier. The defective tube must be replaced. In practice, two new tubes are installed, and the old ones tested by restoring them one at a time later on, when no audience is present.
(28) A short-circuited filter coil will cause hum; so will an open-circuited filter condenser. A short-circuited filter condenser will burn out the fuses F-l and F-2, shown in Figure 115 alongside the tubes.
Mercury-Vapor Rectifier Tube
(29) A gas-filled tube is capable of rectifying heavier current than a vacuum tube of the same size. The gas commonly used for this purpose is mercury vapor. When the tube is cold drops of mercury can be seen clinging to the inner surface of the glass. The liquid metal vaporizes or boils when the filament is heated.
The action of the vapor is approximately as follows: negatrons from the heated filament "collide" with atoms of the gas, thus ionizing them. In greater detail, what happens is that occasionally one of the outermost negatrons of a mercury atom will find itself powerfully repelled by the field of an emitted negatron, which is approaching rapidly from the direction of the filament. Under some circumstances the repulsive force acting upon atomic negatron will be strong enough to drive it away from its atom. The remainder of the atom then constitutes a positive ion, while the detached negatron goes on to the positive plate precisely as if it had been emitted by the filament. Therefore at any given moment during its operation a mercurcy-vapor tube contains numbers of positive ions of mercury, which assist the emission from the filament, in the following manner :
It must be remembered (see Page 452) that an emitting cathode is surrounded by a space charge. When the cathode is in the same tube with a positive plate, some of the negatrons that make up the space charge are drawn to the plate. Some negatrons, however, are emitted with such feeble velocity that they never travel far enough from the cathode to respond to the attraction of the distant plate, but return to the cathode instead. Thus the