Richardson's handbook of projection (1930)

1120 HANDBOOK OF PROJECTION FOR Of course a resistance coil and slider such as is shown in Fig. 406 is not really used. Such an arrangement, while convenient for the purpose of illustrating the operating principle, would be a very poor mechanical device for practical use. In practice the same thing is accomplished in a very much better way by arranging thirty (30) short coils of resistance wire in a circle around a metal disc (G), which represents one side of the circuit, as shown in Fig. 408. Half of these coils are connected in series with each other and in series with the upper circuit at B, Fig. 407, and half connected in series with each other and in series with the lower circuit at B, Fig. 407. In other words, half of them, fifteen in number, are connected in series with the film pick-up amplifier of each motion picture projector. Examining Fig. 408, plate or disc G is connected to one side of the projector pick-up amplifier circuit. It therefore is charged by that circuit with its full voltage. The row of round contact buttons surrounding this disc rest on top of and each one is connected to one of the small resistance coils which, taken as a whole, represent resistance A B, Fig. 406. Arm J receives the current from disc G through sliding contact I, whence it passes through arms H and J to the resistance coil contact, the amount of resistance in series with the pick-up circuit at the moment being dependent upon the position of arm J. Arms H and J correspond and act exactly as would slider C, Fig. 406, insofar as electrical effect be concerned. I think the action should be at least fairly plain to you. Arm F is nothing more than a locating arm. On its under surface, at its end, is a roller which is held