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CURRENT RECTIFICATION 277
and delivers d. c. to the projector light source, (b) It receives current at line voltage and delivers voltage at the arc as low as is consistent with efficient operation. (5) To secure the required stability and steadiness in illumination the supply voltage should be somewhat higher than the arc voltage, and the difference is broken down by what is known as "ballast resistance," which is the name applied to the rheostats used for that purpose. (6) The difference must not be less than fifteen volts, though the exact amount varies according to the character of arc. Any unnecessary pressure means waste of power.
(7) It is quite possible to project motion pictures with an a. c. light source. Many small theatres are doing so, using a special type of a. c. arc made possible by special carbons. This is good practice only for theatres requiring a light source of limited power.
Let us examine the various devices designed to receive alternating current and supply projection light sources with direct current.
The Mercury Arc Rectifier
(8) The mercury arc rectifier was commonly used for current rectification some years ago. It has been very largely displaced by motor generators and rectifiers of other types.
(9) There are many objections to mercury arc rectifiers for motion picture projection. They are limited in capacity. They will not stand for any overloading. Tubes are costly and of uncertain length of life. Some tubes will operate for a long interval; others will burn out, for no discoverable cause, in a few hours. The tubes give off a rather weird light, a pronounced hum, considerable heat and break easily. The d.c. drawn from them is often very irregular.
(10) On the other hand the. mercury arc rectifier is light in weight as compared with a motor-generator set. It is relatively low in first cost. Its efficiency is high — there is relatively little loss through power absorption, though the tube upkeep is considerable. Voltage reduc