SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 259 



250 watts. This will suffice to raise the temperature of the sample 

 at a rate of about 100° C. per second and will cause its rapid de- 

 struction. 



This heating of the specimen by the measuring current was 

 observed on numerous occasions when making tests at 2000 and 

 1000 volts, and in each case the samples on removal from the 

 furnace were found to contain one or more spots where the por- 

 celain had been fused into a glass by the intense local heating. 

 In the later work at lower voltage, this effect was not present, 

 and readings were taken only when the current was substantially 

 constant. 



POLARIZATION 



The early measurements with high voltage direct current 

 showed a number of puzzling discrepancies, such as a variation 

 of the apparent resistance with the voltage used in making the 

 measurement and with the time of application of this voltage. 

 Such discrepancies indicated the presence of an additional phe- 

 nomenon to be reckoned with, which in the absence of definite 

 knowledge as to its origin was called "polarization" and will 

 be so referred to throughout this report. 



The fundamental manifestation of this so-called poliarization 

 is that if a constant D. C. voltage be applied to a specimen, the 

 resulting current will decrease at first rapidly, and then more 

 gradually. The reduction in current is often equivalent to an 

 increase in resistance by a factor of 10 or 20. If the specimen is 

 allowed to remain at a high temperature but without applied 

 voltage for some time, the effect gradually disappears, but a 

 considerable time is required to accomplish this. The disappear- 

 ance is more rapid at high temperatures than at low. Figure 3 

 gives a record of the variation of the apparent resistance of a 

 glass beaker, as measured with 1000 volts D. C. after various ap- 

 phcations and removals of the measuring potential. The course 

 of the experiment is indicated by the arrows and the duration of 

 each period of application or of rest is indicated on the curve. 

 The lowest and highest curves give the resistance as observed 

 with very short application of the testing voltage just prior to the 



