A clock-controlled tuning fork as a source of constant frequency

Note: The art of electrical communication employs such a wide variety of methods for the transmission of intelligence that it utilizes alternating currents whose frequencies cover the entire range between a few cycles per second and several million. With the increasing use of these methods, it becomes more and more imperative that determinations of the frequency of any alternating current may be made with extreme accuracy. In particular, recent developments in carrier current telephony and telegraphy over wires have placed exceedingly rigorous limits on the frequency adjustment of certain types of apparatus. It is many times necessary to hold such equipment as oscillators or filters to within 0.1 per cent. of given frequency values under commercial operating conditions. This means that calibrating devices used in the manufacture and maintenance of such circuits must be reliable to 0.01 per cent. and that the primary standard should be good to about 0.001 per cent. or one part in 100,000. The present paper discusses one of the methods recently developed in the Bell System Laboratory for obtaining a source of practically constant frequency with which other frequencies may be compared. It consists of a clock-controlled tuning fork making 50 vibrations per second and, as is shown, the maximum deviation of its frequency from the mean is less than one part in 50,000. A study has also been made of means for improving the constancy of the control clock and a new type of clock mechanism consisting of an electrically actuated pendulum, the impulse of which is controlled by a photo-electric cell, is suggested. — Editor.