MEASUREMENT OF INDUCTANCE 



385 



in mind the different values of K agree with each other very closely. 

 It has already been stated that r^ should be small compared with X 

 and therefore the values of K using ri = 828 ohms would not normally 

 have been used for the measurement of this coil. 



Table III gives a comparison of the inductance of several coils as 

 measured on the Owen bridge and by a resonant method, the last 

 column giving the difference between the two methods in per cent. 



TABLE III 

 Comparison of Owen Bridge with Resonance Bridge 



The resonant method was a highly accurate one in which frequency 

 errors were negligible. The accuracy was probably of the same order 

 as the measurements on the Owen bridge. The agreement between 

 these two methods does not in itself indicate the accuracy of either 

 method. However, the resonant measurements were made on a 

 completely shielded equal ratio-arm bridge,^ in terms of frequency 

 and capacitance, using entirely different equipment from the Owen 

 bridge in which the inductance is measured in terms of resistance and 

 capacitance. Accordingly it is very improbable that these two 

 methods had any errors in common and we may assume that the 

 agreement obtained is a fair measure of the combined error of the 

 two methods. Consequently from this table we see that for a range 

 of .1 to 3 henrys and for frequencies up to 2,000 cycles the error in 

 the measurement of the inductance by the shielded Owen bridge is 

 less than .1 per cent and for 10 henrys is less than 1/4 per cent. 



Accuracy — Measurement of Resistance 



The measurement of effective resistance in the case of an impedance 

 of low reactance practically consists of the substitution of the unknown 

 for the known resistance. In this case the accuracy of the measure- 



