A Mechanically Resonant Transformer

A transformer for use in the audio-frequency band is described whose output potential is remarkably selective to frequency. In its simplest form the diaphragm of a telephone receiver is replaced by a tuned reed of magnetic material. When a frequency is applied to the receiver corresponding to the natural frequency of the reed it is set in vibration and its motion is employed to generate potentials in another receiver unit adjacent to the tuned reed. A typical transformer had a selectivity of 115 at a frequency of 1500 cycles.