THERMAL CONDUCTIVITIES OF SOME ELECTRICAL INSULATORS. 453 



If ', nnd /, air the temperatures and R, and Rj the resistances of the two coils 

 between AC and CB respectively, in the first case (t r > t t ) we have 



005109 

 ',= % 



and l\('-i in the second (t' i >t' l ) we have 



-264-9 



' 

 Therefore 



Ui-Ri = -005109/, - -005084*, + 264'9 ('005 !09)-264'6 ('005084), 



R'a-R, = -005109', + '005084',-264'9 ('005109)4- 264'6 ('005084). 

 Hence 



',-R, = '005109 (t l -t\)+ '005084 (t' t -t,), 

 or, since t l t a = t't'\ nearly, 



t 



2 2 



or 



. . ,.,,. ., Mean difference of resistance 



Mean temperature dinerence between coils = - - , 



I M I.)U Jt) 



or 



Mean A/ = 196 '2 mean All. 



In addition to the difference of temperature of the two coils we require, if the 

 conductivity varies with temperature, to know the mean temperature of the material 

 between the two points at which the difference is measured. 



Since the conductivity does not appear to change very rapidly with temj>erature, 

 this mean temperature need not be determined with the same degree of accuracy as 

 the temperature difference. To determine this mean temperature, the temperature of 

 the hotter of the two coils used in measuring the difference of temperature was 

 determined by making it one arm of a resistance bridge, of which two other arms had 

 a constant ratio, and the fourth was adjustable. The resistances of the fixed arms 

 were 4'850 and 8'535 ohms respectively, and the adjustable arm (r) had therefore 

 when a balance was produced 1762 times the resistance of the coil to be measured. 



Since the two coils used in the difference measurement are nearly alike in resistance, 

 it will IK; sufficient to make the calculation as if they were each equal to their mean. 



