SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 265 



obtain a balance on the bridge is a serious draw-back because of 

 the rapid change in the resistance to be measured with even sUght 

 drifts of temperature. 



The megger, while extremely rapid and convenient, is open 

 to the disadvantages of polarization and to the fact that the 

 voltage supplied varies very considerably with the resistance of 

 the specimen under test. 



The use of a magneto in place of alternating current as a source 

 has the great advantage that it approximately duplicates the 

 conditions of operation in the engine. The magneto, however, 

 is very variable in its output, both from instant to instant and 

 as a result of permanent changes in the magnets, contact points, 

 etc. Moreover, there is an abrupt change in the operation of 

 the machine when the resistance of the specimen becomes so 

 low as to cause the spark in the safety gap to cease, and also 

 the total variation of the current delivered with various resistances 

 in the circuit is comparatively slight, with this type of machine. 



A method involving the measurement of the rate of loss of 

 charge from a condenser connected in parallel with the specimen 

 has been used by Cunningham. This method imitates the con- 

 ditions of operation much more closely than does the A. C. 

 method but not as perfectly as the use of a magneto as a source. 

 The principal objections are the very delicate string electrometer 

 which is required and the necessity of recording the result 

 photographically . 



TYPICAL RESULTS AND CONCLUSION 



The following table gives the results obtained by the use 

 of the alternating current method on a number of types of sam- 

 ples, the significance of the various constants being the same as 

 those defined on page 257. 



These figures show a wide variation in the resistance of the 

 different materials but a rather surprising similarity in the con- 

 stant b which is a measure of the temperature coefficient of their 

 resistance. It should be mentioned in this connection that while 

 successive measurements with alternating current on a single 

 specimen give results repeating to a few percent, yet measure- 



