Richardson's handbook of projection (1930)

1128 HANDBOOK OF PROJECTION FOR "shaky." However, whatever plan be adopted to regulate and stabilize the projection speed for sound, there must be provision for suitable alteration in speed when silent pictures are shown. The Motor Control Box, in conjunction with the special motor employed to drive motion picture projectors where Western Electric sound apparatus is used, is designed to and does take care of the requirements above set forth, it having been decided that neither compound wound or synchronous motors would provide sufficiently close regulation of speed under the common voltage fluctuations and frequency fluctuations found in commercial power circuits supplying theatres. Lack of strong starting torque of synchronous motors was another objection. The Motor Control Box depends for its operation upon what is known as a "tuned circuit," also called a "resonant circuit." In layman language this means a circuit which will permit the free passage of alternating current of a certain frequency, but will offer high resistance to current of any other frequency. The frequency such a circuit will pass is called a "resonant," a "tuned" or a "critical" frequency. A simple demonstration of this phenomenon may be made as follows : cut an ordinary rubber band, such as we use to hold papers together, in two and, holding one end, attach a weight sufficient to stretch it somewhat to the other end. It will be found that if the weight be started bobbing up and down, so long as the movement is regularly timed, it will require slight effort on your part to maintain the regular up and down action of the weight. If, however, you alter the speed in any degree, you will discover that you have upset the rhythm, and