A New Tone Generator

New Pure Tone Generator and Receiver of Sounds.—(1) Construction and operation. The instrument consists of a thin, non-magnetic, metallic diaphragm between two flat coils through which a constant direct current $I_0$ flows in such a way as to produce a radial magnetic field in the diaphragm; then when a simple harmonic alternating current $I$ of the frequency $\frac{\omega }{2\pi }$ is superposed upon the direct current, circular currents are induced in the diaphragm, which thereupon is acted upon by a simple harmonic electrodynamic force and vibrates with the frequency of the alternating current. For low frequencies the electrodynamic force is approximately proportional to $\omega I_{0}I sin (\omega t+\theta )$ and the amplitude of vibration is approximately proportional to $\frac{I_{0}I}{\omega }$. The absence of overtones is due to the absence of ferromagnetic material, and to the fact that the radial magnetic field is constant. The aperiodicity of the diaphragm renders the calculation of the performance of the instrument practicable, and eliminates distorsion, due to resonance, in the wave form of the emitted sound when the instrument is excited by a complex alternating current. When used as a generator of pure tones, the coils were connected in the circuit of a thermionic oscillator whose frequency could be varied from 500 to 25,000 vibrations per second. When used as a receiver of sound, the current generated in the coils by the motion of the diaphragm is fed into a thermionic amplifier. (2) Quantitative study of the performance. The distribution of the magnetic field between the coils was determined experimentally; the diaphragm current equations were deduced and solved for a particular case; the forces on various parts of the diaphragm were calculated, and thence the amplitude of vibration and the sound energy output. With an aluminum diaphragm 0.0025 cm. thick and 10 cm. in diameter, a direct current of 1 ampere, an alternating current of 0.085 ampere, and a frequency of $\frac{{10}^5}{2\pi }$, these were respectively 7 × ${10}^{-7\mathrm{}}$ cm., and 9 ergs per second. By increasing both direct and alternating currents five-fold, the output could be increased over six hundred-fold. Measurements of the amplitude for various frequencies agreed well with the calculated values. (3) Applications of the instrument. Since it gives a pure tone of constant and measurable pitch and intensity over a wide range, it would serve as a precision source of sound, useful both for research and lecture purposes. When used as a telephone receiver and transmitter, actual tests have shown that the reproduction of sound is remarkably faithful.