Signedon instrument: CENTRAL SCIENTIFIC CO.
FunctionThis device is for demonstrating the principle of the Frahm vibrating reed tachometer. Such an instrument is used to measure the number of revolution per minute. The reeds all have different vibrational frequencies. The rotor is purposely unbalanced so that when it spins, it communicates to the frame a vibration rate equal to its rate of rotation. The steel reed whose natural period of vibration agrees with the rate of rotation of the rotor is set into sympathetic vibration, its end moving through an arc of about 2.5 cm.
An early twentieth-century description of a Frahm tachometer is found on the following website.
Primary SourcesCentral Scientific Company, General Catalog of Laboratory Apparatus and Scientific Instruments [J-141] (Chicago: Central Scientific Company Laboratory Supplies, 1941), 1411, no. 84920.