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 <i>n</i><sup>th</sup> harmonic of the radio frequency oscillator the steps of frequency may be made 1/<i>n</i> 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/<i>n</i> 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)