A direct-reading frequency meter of long range
Albert Campbell · Proceedings of the Physical Society of London · 1924
A direct-reading frequency meter for audio frequencies is described. It is a null instrument, reading by single adjustment, the working system embodying a new method of balancing mutual inductance by resistance. The standard type has five ranges, covering from 180 to 4,000 cycles per second, with accuracy of the order of 1 in 1 000, and negligible temperature coefficient except at the lowest frequencies; an extra set of ranges from 18 cycles per second upwards can be easily added.