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568 HANDBOOK OF PROJECTION FOR
from reversing its direction and passing into the graphite terminals.
The A. C. supply circuit is connected to graphite terminals A and Al, Fig. 183, through an auto-transformer, which lowers or raises the voltage to that required at the arc, and as the above described action will only allow current to flow in one direction, the pulsations of current which pass alternately from terminal A and Al, Fig. 183, into the mercury vapor must, of necessity, all pass out of the vapor through mercury terminal B, Fig. 183, which is connected to the arc lamp. As a result the arc receives a continuous, slightly pulsating current which differs but little from ordinary D. C. Ordinarily the pulsations would be quite pronounced, but this is prevented by a feature of the auto-transformer (main reactance) which decreases them to such an extent that the current delivered at the arc has a very nearly constant potential value.
Before the bulb starts working it contains no mercury vapor. Within the bulb is a vacuum which must be filled with mercury vapor before current can flow. Once the space is filled with mercury vapor, however, and current flow has been started, it will continue to flow as long as it is uninterrupted, but any interruption, even for the shortest period of time, permits the vacuum to re-establish itself and stops the operation of the bulb.
HOW THE BULB IS STARTED.— In order to fill the bulb with mercury vapor, it is tilted until the mercury in terminal?, B, C, comes into contact, and since terminals B and C have direct connection with the A. C. supply, through a special circuit, current flows between terminals B and C. The tube is then rocked back to upright position which breaks the mercury bridge thus formed between terminals B and C, and in breaking it forms an arc or spark, which creates the initial current-carrying mercury vapor, and puts the tube into operation.
The alternating current supply circuit is connected to an auto-transformer, or main reactance, the terminals of which are conected to the terminals A, Al, Fig. 183. From terminal B the current passes through the projector arc, and the circuit is completed through a connection to the middle point of the autc-transformer.
The principal parts of a rectifier are : (A) an auto-transformer; (B) a regulating reactance coil; (C) a tilting mechanism; (D) a relay; (E) a dial switch; (F) a switch or other