SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 257 



in ignition systems, and although this source of voltage was later 

 abandoned, the data obtained with it brought out the salient 

 facts in regard to this type of conduction. The most striking of 

 these facts, as verified by other measurements made later, is the 

 very rapid decrease in resistance of the specimen with increase 

 in temperatm-e. This variation amounts to approximately 

 2 per cent per degree Centigrade at all temperatures. If the re- 

 sults are expressed by plotting resistance against temperature, 

 or conductance against temperature the resulting curves are so 

 steep as to render it impracticable to express the data over an 

 extended temperature range by a single curve. It is found, how- 

 ever, that by plotting the common logarithm of the resistance 

 against temperature, as in figure 2, a convenient Hne of sUght 

 curvature is obtained, and if this curvature is neglected, the 

 results can be represented approximately by the equation 



logio R = a — bt (3) 



This method of expressing the results is very convenient in re- 

 ducing the data to a basis of resistivity, since combining the re- 

 lation 



p = KXR (4) 



with equation (3) one obtains 



logio p = a + logio K — b t = c—bt (5) 



In this equation b and c are constants of the material and are 

 independent of the shape and size of the specimen used. Un- 

 fortunately, however, the values obtained for one of these con- 

 stants depends very markedly upon the other, so that a slight 

 error in one will cause a compensating change in the other. They 

 are, therefore, not well suited for comparing the relative merits 

 of the different materials and for this latter purpose it has been 

 found convenient to compute an "effective temperature" (Te), 

 which is defined as the temperature at which the material has the 

 arbitrarily selected resistivity of one megohm per centimeter cube, 

 and which may be computed from the equation 



c - 6 



T. = ~r- (6) 



