Richardson's handbook of projection (1930)

1130 HANDBOOK OF PROJECTION FOR the rubber will give slight aid in bobbing the weight -until you again settle to the former rate of speed. In other words the mechanical arrangement is such that it will permit of back-andforth movement at a certain sustained rate of frequency, but will offer resistance to other frequencies, which is analogous to the action of a tuned electrical circuit. It is also very similar to the action of a tuning fork, which will vibrate only at the speed it was designed to act at. In fact, any combination of mass and elasticity will act much the same way. And now we arrive at the meat of this particular nut. A choke coil or an inductance has the same effect upon an alternating circuit that a weight has upon a rubber band, or as a flywheel has upon a mechanical movement, and a condenser (See page 1017) plays the same part electrically that our rubber band does mechanically. Alternating current is nothing but a back-andforth movement of electricity in a circuit, hence by the use of a choke coil and a condenser it is possible to set up or create resonance effects. Basically that is all there is to a "tuned circuit." In stabilizing motor speed the first requirement is that the motor must automatically "advise" the control box of any variation in its speed. This has, in this case, been effectually accomplished by building into the motor a small A. C. generator, located on the motor armature shaft and covered by the same housing. Figs. 411 and 412 show the plan applied to both an A. C. and a D. C. motor, in simplified form. This small generator will generate A. C. whenever the projector driving motor is in operation, and the fre