Abstract
The characteristics of telephone receivers are now well known as electric motors. They are, however, here studied as electric generators driven by sound. Necessary formulae for such vibratory generators, essentially borrowed from the theory of receivers, are first mentioned.
Experiments were carried out with two receivers, coupled acoustically with a tube, one as a motor and the other as a generator. The induced electromotive force of the generator varied remarkably with the frequency, although the motor current was kept constant. It reached, in one instane, up to 57 millivolts for 1 milliampere of motor current. Such a variation occured because the apparatus used in the tests was a coupled system of two diaphragms through an acoustic tube, each having its own natural frequency or frequencies. The e.m.f. also varied somewhat in the similar manner when the length of coupling was changed, keeping the frequency constant.
When the generator was loaded with capacitance, the terminal voltgae incr ased at first with the load current and then decreased. The current thus taken out, reached to 204 microamperes in one instance. For resistance load, the voltage dropped from the beginning with increase of load current. Thus the vibratory generator showed the same characteristies as alternators.
In addition, a system of two receivers with an acoustic coupling as used in the present tests, is studied in the appendix.
