Measurement of Frequency

Absolute measurement of frequency.-Various methods of absolute measurement of frequency are first described. Particular attention is devoted to the stroboscopic method and the phonic motor method for audio frequencies. A modified method of harmonic comparison is also presented. Standard frequency oscillators.-The author has designed a clock-controlled oscillator which consists of a tube-driven 25-cycle adjustable tuning fork and a chronometer. The performances of various kinds of tuning fork oscillators are discussed mathematically, and a new type, which has a magnetic device of such nature as to make the electromagnetic controlling force extremely small, is described. The factors affecting the constancy of frequency of quartz oscillators are discussed. Resonators.-Several types of electrical and mechanical resonator are compared with respect to their usefulness as frequency standards. For expressing the sharpness of resonance, the author recommends the adoption of the quantity A defined by the equation A = ω
0
/2δ, where wo is the angular velocity at resonance and δ the damping constant. Experimental results are presented showing the sharpness of resonance of a number of resonators of different types.