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588 RICHARDSON'S BLUEBOOK OF PROJECTION
dously). Moderately smooth d. c. is all that is required for speaker fields, and such current can be obtained from any of a number of convenient sources.
We saw in Figure 139 that the field winding of a loud speaker could be made to do double duty by acting also as a filter choke in the output circuit of an amplifier's rectifier. This method of powering the speaker avoids the necessity for a separate source of d. c. and saves the cost of an additional part for the filter. This is the most modern method of providing speaker field supply, and is used in nearly all the more economically-priced systems.
A second method, one of the earliest used, is to power the field coil by means of a bank of storage batteries.
A third procedure, very widely followed, is to use either line d. c, or, where only a. c. lines are available, the d. c. output of the arc generator or rectifier. This method of power supply usually requires a resistance to reduce the d. c. voltage to the voltage needed. To make the power loss in that resistance as small as possible speaker fields are often wired in series. The resistance commonly takes the form of a rheostat, which can be adjusted to compensate for voltage variations in the supply source. Such a rheostat, together with an ammeter that is consulted in adjusting it, is often mounted in a metal box, the entire assembly being called a "horn field control cabinet," or by some similar name. Switches are usually provided backstage, by means of which any speaker field that may open-circuit can be shorted out at once ; otherwise a break in one field winding of the series circuit would stop the show until the faulty unit had been replaced. The horn field control cabinet, mounted in the projection room, contains a similar switch to guard against open-circuit in the monitor.
A fourth method of powering speakers is by use of rectifiers which may be either of tube or of disc type. The combination vacuum tube rectifier of Figure 115 has an output circuit (lower left) for speaker field supply. Individual rectifiers are also used for the purpose. In some cases these are mounted backstage, and are under care of the stage crew rather than of the projectionists.