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504 RICHARDSON'S BLUEBOOK OF PROJECTION
projector No. 1 is turned off excessive current might flow in the other apparatus supplied by the rectifier if S-l, in co-operation with R-5, did not serve as protection. S-l is in series with the supply line to projector No. 1. When that projector is operating S-l acts as an electro-magnet, pulling the hinged bar just above it away from the arrowhead contact. When the circuit of projector No. 1 is opened, current no longer flows through S-l. The hinged bar springs back and makes contact with the arrowhead. The current supplied then flows through R-5, which thus serves as a substitute load whenever projector No. 1 presents an open circuit.
This arrangement protects other apparatus in case of an accidental open circuit in projector No. 1. It also permits that projector's exciting lamp and photo-cell amplifier to be turned off whenever not in use, thus lengthening the life of the lamp and of the amplifier's tubes.
Directly to the right of the filter-supply to projector No. 1 is a similar arrangement supplying the other projector. This last circuit, however, is slightly different. A line branches off from the second filter stage and runs through a third filter, which has its own voltage regulating rheostat and bleeder. This third filter, shown in the lower right hand corner of the drawing, supplies a quarter of an ampere to the filaments of the vacuum tubes in the board, or system, amplifier. It has no protective relay since the system amplifier is not turned off separately.
At the lower left of the drawing is still another output circuit, with only a single stage filter consisting of one inductance and one condenser. This supplies magnetizing current to the field windings of the loud speakers and (24) does not need much filtering because a. c. hum in this circuit will not be amplified.
The 24-volt d. c. terminals at the bottom of the drawing can be used, if desired, with or without an external filter, for still other d. c. purposes. For example, if properly filtered they could supply microphone power.
The circuit of Figure 115 is identical in principle to the