F. H. Richardson's bluebook of projection (1935)

492 RICHARDSON'S BLUEBOOK OF PROJECTION without the help of a filtering condenser, and often with the help of a rheostat to reduce the voltage to that required by the speaker, or by a number of speakers connected in series. The arc lamp d. c. supply from a rectifier or motor-generator can be, and sometimes is, used in the same way. Motor Generators One method of providing for the varied d. c. requirements listed above is to install a number of generators, which may be driven by either a. c. or d. c. motors, according to the power supply available. (The fundamental principle of motor-generator action, and some details of motor-generator construction, will be found elsewhere in this volume. It does not seem advisable to go into further detail here. Books containing almost endless information about motors and generators can be found in any public library, or may be obtained from any publisher of technical books. No space that could be allotted here could do the subject equal justice. To attempt to include such details would only take that much space from matters peculiar to the projection craft, concerning which very little information is generally available. ) As used in the projection room for sound power purposes, motor-generators are usually small, light machines that can be switched on and off without necessity for any starting rheostat. (6) They are generally equipped with filters to eliminate commutator ripple. Filter, voltage regulating resistance, switches and fuses may be mounted in one "generator control cabinet" or may be distributed in several cabinets or boxes. In any case the connections represent only commonplace electrical circuits, and the projectionist should have little trouble in tracing and understanding those with which his own projection room is quipped. Tracing Circuits (7) It is always advisable to study an unknown circuit by means of a schematic, or skeleton drawing, which