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RCA PHOTOPHONE SYSTEM
351
mented. This vibrator is then placed between the poles of a permanent magnet in such a way that the wires lie across the plane of the magnetic flux. You will notice that we have here all the elements of a simple motor. If this loop were free to
Vibrator tension scale 5 divisions
Vibrator tension "adjustment
LocKing screw for "galvanometer vibrator
Screw C for adjustment of rotatinq mirror
v — ^At-V.V^AMP
Screws for adjusting position of sound tracK
adjusting position of sound tracK
FIGURE 2 Galvanometer and Optical System.
turn, it would rotate like the armature of a motor. Being tied down tightly on each end, however, it vibrates when the alternating current signal from the amplifier passes through it. This vibration about a vertical axis occurs in that portion of a loop between the two ivory bridges and the little mirror, being firmly attached to both sides of the loop, vibrates also.
The tension applied to these wires determines their natural period. By natural period, I mean the point at which the vibrator will give the greatest response for a certain current input. All vibrating bodies have a natural period or resonance point and this device is no exception to the rule. Its period is approximately 6000 cycles. The peak in the case of this vibrator is a comparatively small one, being of the order of perhaps five to six db. This over-response is not bad enough to cause a great deal of distortion in itself, but its action is further