258 SILrSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 



This value of T^ ranges from 350° C. in the poorer grades of 

 porcelain up to 800° C. for fused silica, and is a convenient index 

 of the value of the material as an insulator at high temperatures. 

 There is a complete absence of any critical temperature at which 

 the material undergoes an abrupt change in its resistance. This 

 shows the error of the commonly accepted idea that porcelain 

 breaks down and becomes conducting at a definite temperature. 

 This belief probably originated from experiments in which the 

 temperature of a porcelain sample was gradually raised while 

 being continuously subjected to an applied voltage. The effect 

 of the current flowing through the sample in such cases would 

 be entirely negligible up to a certain temperature at which the 



power, ~ , supplied by the measuring current, became comparable 



with the rate at which heat could be dissipated to the surround- 

 ings. Owing to the very rapid rate of change of resistance with 

 temperature, a very slight further increase in temperature would 

 materially decrease the resistance and consequently increase the 



E^ 



^ loss. Unless the specimen was in a position to give off heat 



freely to its surroundings, the temperature would rise rapidly 

 causing a further decrease in resistance, thus leading to an un- 

 stable state which would rapidly cause the fusion of the material 

 and the passage of an arc. The rapidity of change of resistance 

 with temperature makes this point of instability quite definite, 

 provided all the conditions of the experiment are maintained 

 constant, but this apparent critical temperature will depend very 

 greatly upon the contact between the specimen and the furnace, 

 upon the applied voltage and other conditions, so that this is 

 in no sense a specific property of the material. 



The magnitude of this heating effect is exemplified by the 

 behavior of a porcelain sample tested when hot, for example at 

 500° C. At this temperature, the resistance of a centimeter cube 

 of ordinary porcelain is about 100,000 ohms, and if a voltage of 

 only 500 volts per millimeter (i. e., only about V20 of that required 

 to puncture it while cold) be applied, the current flowing will be 

 50 milliamperes and the power dissipated in the sample will be 



