A Direct Reading Frequency Bridge for the Audio Range, Based on Hay´s Bridge Circuit

This paper describes the construction and operation of a direct reading bridge for measuring audio frequencies in the range from 50 to 5000 cycles per second and upwards. The circuit used is a modification of Hay´s bridge for the measurement of inductance, and is obtained from it by the insertion of a variable resistance in the inductive arm. This resistance and another in the condenser arm are varied for balance. For the design as given, the frequency in cycles per second is determined by the number of ohms in the variable resistance plus that of the internal resistance of the coil in series with it. The bridge is portable, simple to construct, and makes use of external connection for the variable resistances to two standard decade non-inductive resistance boxes. Its operation is convenient and permits of a precision of balance of 0.1 per cent with a probable error of less than about 0.25 per cent.