A new method for the absolute determination of frequency

The new method for the absolute determination of acoustical frequencies described in this paper possesses several useful features. Among these are: ( a ) the rapidity with which very accurate results may be obtained by its use, ( b ) its practical convenience and simplicity, ( c ) the inexpensiveness of the apparatus required, and ( d ) its availability for use with any of the standards of time ordinarily available in a physical laboratory, i. e ., a “seconds” or “half-seconds” pendulum clock, a “half-seconds” chronometer, or even merely a good stop-watch. The method is the outcome of an investigation undertaken at my suggestion by Mr. Dey, and its value has been proved in an extensive series of tests carried out by him. The principle on which the method is based is that of the maintenance of oscillations of sub-synchronous frequency by a periodic field of force, and is applied in practice in the following manner:— A pendulum formed of an iron rod 0.5 cm. diameter hangs vertically from a simple wire-hook suspension. A brass bob slides on the pendulum and is capable of being fixed in any desired position. The length of the pendulum is either 35 cm. or 100 cm., so that with the bob in a suitable position the free period of oscillation is roughly either one second or two seconds as desired.