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634 RICHARDSON'S BLUEBOOK OF PROJECTION
or any other means that would reverse the phase of the feedback in Figure 201 would cause the amplifier there depicted to generate alternating current. The frequency generated would depend on the inductance and capacitance of the associated circuits.
(5) If an ordinary old-fashioned telephone receiver is held close to the telephone mouthpiece the telephone will emit a loud squeal. In public address work if the microphone is brought too close to the loud speaker the whole system will emit nothing but a loud squeal. If the phase of the feedback of Figure 201 were reversed the sound system incorporating that amplifier would emit only an intense howl. The principle is the same in every case. If the output is brought back to the input and if the loss in such feedback is less than the gain in the amplifier, oscillation, or the generation of alternating current, results. In any three-element tube if the plate circuit is coupled, inductively or otherwise, to the grid circuit, and if the loss in such coupling is less than the gain or amplification of the tube, the tube becomes the generator of alternating current. It is then spoken of as an oscillator tube. The frequency of the current thus generated depends, as said, on the inductance and capacitance of the circuit. In broadcasting stations one of the condensers in the oscillating circuit is built with a carefully ground quartz plate as its insulating medium. Like the Rochelle Salt crystals referred to in Chapter XX, such a quartz crystal vibrates when alternating current is impressed upon the plates of the condenser of which it is a part, but the quartz crystal can vibrate at only one frequency — that for which it is ground. Hence this crystal-condenser permits the oscillator tube in the broadcast station to generate only one frequency. The crystal-condenser will, however, vibrate at a different frequency if its temperature changes ; it is therefore kept in a small, electrically heated compartment where temperature is accurately controlled by a thermostat.
(6) The output of the oscillator tube, which is a.c. of the frequency the station is authorized to broadcast,