A Bridge for the Measurement of Self-Induction in Terms of Capacity and Resistance
David Owen · Proceedings of the Physical Society of London · 1914
An alternate-current bridge method is proposed for the determination of self-induction in terms of capacity and resistance. The inductance L is given by the relation L = K3r1R; in addition to which it is also necessary for balance of the bridge to satisfy the condition K3r1 = K4r2. The two conditions of balance may be secured in practice without mutual interference. The end point is therefore rapidly attained. The possibility of effecting a balance is unlimited by the value of the unknown L. The method is independent of frequency. This has the accompanying advantage that it is unnecessary to employ a pure sine voltage. Very good results may be attained with a buzzer as source and a telephone receiver as detector, at, say, 500 similar. The dependence of sensibility of the bridge on the frequency is discussed, and it is shown that over a wide range (say, 100 similar to 1,000 similar) the sensibility is high. The effects of residual inductance in the resistance coils and leads, and of absorption in the condensers, may be simply and correctly allowed for, the formula then becoming L = K3r1(R - R0). This fact is of especial importance in the accurate determination of very small inductances. Tests are quoted showing that with the same pair of condensers measurements over the full range from one microhenry upwards may be made. For inductances of the order of 10 microhenries the error may be kept within a few parts in 1,000; whilst if the inductance is as high as a few millihenries the error of any measurement may be reduced below one part in 10,000. The application of this bridge to the determination of capacity in terms of self-inductance is discussed, and an example is given of a test of the temperature variation of capacity of a standard mica condenser over the range 0° - 30°C.