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286 RICHARDSON'S BLUEBOOK OF PROJECTION
(above the line) used by the half wave rectifier. C is the full wave a. c. and D the resultant d. c. wave of the full wave rectifier.
Whenever a high amperage, or a current of exceptional smoothness, is required, it is possible to use a 3-phase rectifier which has six tubes. It is the circuit shown in Fig. 74B repeated three times — once for each of the phases of a 3-phase transformer, the d. c. outputs being wired in multiple.
Types of Rectifying Tubes
(39) Tubes used for arc light source supply rectifiers may be either of the vacuum or gas-filled type. (40) The presence of some gases increases the current flow
4
TOARCL/NE
Figure 74D
through the tube by increasing emission of the negative electrons from the filament. Tubes of 5, 10 and 15, and 30 ampere "space current" capacity are used in these rectifiers.
Maintenance and Care
(41 ) Tube rectifiers such as we have described have no moving parts, so there is nothing except the tubes themselves to wear out or cause trouble. Sometimes — but rarely — the transformer may burn out through overheating. Usually the primary coil is tapped to permit compensation for high or low line voltage. (42) Their temperature should be checked occasionally and if found to be consistently high, proper compensation should be made to reduce primary voltage. Usually a tap switch is provided for the purpose.
(43) If either tube shown in Fig. 74B should burn out, the rectifier will not stop working entirely but will continue to function as a half-wave rectifier. Screen illumination will be cut down by one-half and the light will start to flicker. Tubes rarely burn out but do de