Abstract :
The precision with which standards of inductance can be measured and used at radio frequencies is limited, not only by the ordinary uncertainties of experimental technique, but also by the fact that the standards are necessarily circuit elements with terminations, while inductance is primarily a characteristic of complete circuits. The procedure followed at the National Physical Laboratory when the highest possible precision is required is described in both its experimental and theoretical aspects. The precise relation between the familiar equivalent network for a coil and the basic definitions is indicated, and practical details are given for the resonance method of measurement that has been adopted as standard practice for all work of this kind. It is successfully operated at all frequencies from the audio range, where it overlaps the bridge methods employed as standard practice in the lower ranges of frequency, to values little short of those of self-resonance of the coils. In the overlapping region the bridge and resonance methods agree to about 1 part in 104 and with suitable elaboration the resonance method gives about the same accuracy for coils of any value between 1 H and 10 ¿H at any frequency within their working ranges, the limit for the smaller coils being about 4 Mc/s. With coils of the order of 1 ¿H measured values may show a standard deviation as small as 0.0001 ¿H, but from the general considerations outlined in the paper limits closer than 0.0005 ¿H will seldom be significant in any work with lumped circuits.
