Simple and rapid methods for adjusting the frequency of a radio frequency oscillator in small steps of frequency by aural methods are described. By use of the nth harmonic of the radio frequency oscillator the steps of frequency may be made 1/n times a convenient fixed audio frequency, such as that of a tuning fork. By elaboration of the equipment the adjustments may be made to within 1/n cycles per second, where n is the order number of the oscillator harmonic employed.
J. K. CLAPP, "A NOTE ON METHODS OF RAPIDLY ADJUSTING A RADIO FREQUENCY OSCILLATOR IN SMALL STEPS OF FREQUENCY," J. Opt. Soc. Am. 17, 132-134 (1928)