SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 255 



stantan thermocouples, one of which was inserted in a closed 

 porcelain tube which dipped below the surface of the solder in 

 the interior of the cup, while the other was imbedded in the steel 

 cup containing the solder below the specimen. Readings of the 

 resistance were taken only when these two thermocouples showed 

 substantially equal temperatures. 



In cases where cup specimens were not available, measure- 

 ments were made on assembled spark plugs and also on spark 

 plug insulators, and on short pieces of tubing. In these cases the 

 conduction took place between a central electrode and either the 

 shell of the spark plug or a band of platinum deposited around 

 the center of the outside of the insulator or tube. The measure- 

 ments with this type of specimen were definite in indicating the 

 resistance of the specimen, but owing to the uncertainty as to 

 the area of contact and the location of the lines of current flow, 

 it is difficult from such data to compute with accuracy the true 

 resistivity of the material. 



For reducing the results of either type of specimen from the 

 observed resistance to a basis of the resistivity of the material, 

 the factors connecting these two quantities were computed from 

 the dimensions of the specimens. For the cup specimen, 

 the resistivity is obtained by multiplying the observed resistance 

 by K, where 



and d is the diameter of the bottom of the cup and t the thickness 

 of the cup in centimeters. For the tubular specimen 



K = ^ (approx.) (2) 



^2.30 logio^^ 



where / equals the length of the external conducting band meas- 

 ured parallel to the length of the specimen, and Ro and Ri are 

 respectively the external and internal radii of the insulator. 



In most of the work the resistances were measured by reading 

 a voltmeter connected across the specimen and an ammeter in 

 series with it, and taking the quotient of these values as the re- 



