A Precise and Rapid Method of Measuring Frequencies from Five to Two Hundred Cycles Per Second

After a brief discussion of some of the common methods of measuring frequencies from 0 to 200 cycles, a method is described which offers important advantages, combining a high degree of accuracy with ease and rapidity of use. The method depends on the fact that if a condenser be discharged through a resistance f times per second (the condenser being charged to the same initial voltage each time), then the average voltage drop across the discharging resistance is directly proportional to f. The unknown frequency is made to control the number of times per second the condenser is discharged, and thus the voltage drop mentionted above is proportional to the unknown frequency. A circuit arrangement is described whereby this voltage drop is balanced, through a sensitive galvanometer and high resistance, against a known fraction of the total voltage drop along a slide-wire resistance shunted around a storage cell. By first calibrating the system with an alternating current of known frequency, it is possible to read unknown frequencies directly off the slide wire. A discussion of the sources of error, and experimental determinations of the error actually observed, lead to the conclusion that, in the range from 75 to 200 cycles per second, the accuracy is always better than one-tenth cycle.