GROVER AND CURTIS: INDUCTANCE MEASUREMENT- 195 



The standards used were in the form either of parallel wires 

 or of circles. Only in the case of one-ohm coils was it pract icable 

 to use both forms. The agreement between the two forms was 

 satisfactory as is shown by the following example. The differ- 

 ence in the computed values, after allowing for the inductance of 

 the binding posts, was 128 x 10~ 9 henry. The measured differ- 

 ence in the inductance was also 128 x 10 -9 henry. With stand- 

 ards having larger resistance different sizes and lengths of wire 

 were sometimes used, as well as different distances apart. As 

 an example, the 5000-ohm parallel wire standard was measured 

 with the wires 2 cm. apart and again with them 10 cm. apart. 

 The observed difference was 114 microhenrys. The computed 

 difference when taking into account the distributed capacity 

 between the wires was 110 microhenrys. 



While in all cases the standards were adjusted to have nearly the 

 same value of resistance as the coil to be measured, yet it was 

 necessary to be able to vary the resistance thru a small range 

 without varying the inductance. This was accomplished by 

 two different devices. Steps of a tenth of an ohm were obtained 

 by substituting for manganin links, copper links of the same size 

 and form. A continuous variation of resistance from 00 to 0.1 

 ohm was obtained by means of a copper wire sliding in a tube of 

 mercury. The change in inductance was very small in this case, 

 but where small inductances were being measured, it was necessary 

 to make a correction on this account. 



The chief difficulty encountered was with coils of high resist- 

 ance where the capacity between the wires appreciably affects 

 the effective inductance. If L is the computed inductance, C 

 the capacity between the wires and R their resistance, then the 

 effective inductance U is approximately given by the equation 



V = L - hCR 2 . 



In commercial resistance coils having resistances of 100 ohms or 

 over the last term is usually the larger of the two, so that the 

 effective inductance is negative. With coils greater than 1000 

 ohms the actual inductance is usually a negligible part of the 

 effective inductance, so that the value of the effective inductance 

 depends entirely upon the capacity and resistance. With parallel 



