THKUM.M. OINMHVTIVITIKS OF SOMK KLKCTHICAI, INSULATORS. 



i,. 



thermometer, i.e., when the hydrogen thermometer reads 217.* Hence when the 

 platinum thermometei used in this work reads 235, i.e., when the temperature 

 indicated by the hotter spiral is 215 platinum degrees, the resistance of the copper 

 leads in the Dewar tulie will be zero. 



The resistance of the coil itself at any required temperature is best determined 

 from these data graphically, on the assumption that the change of resistance is 

 proportional to the change of temperature (tig. 8). 



L:ods in 

 Air. 



Leads in 



50 



-ijo -220 -aoo -iflo -160 -no -no -too -fto -60 -40 -20 o 



Temperature of Hot Coil , Pt. ScaJe 



Fig. 8. Resistance of heating coils. 



20 



4O 6O 



80 



100 



Tf C amperes are sent through the heating wire, the heat generated in it per 



RO 3 

 second = ~ -^ gramme degrees, and since the length of the wire is 5'fi centims., the 



4 



heat H generated per second per centimetre = 

 But we have found (p. 449) that 



IK - 



o X 4 



H 



Therefore 



k r = '00988 



where R is the resistance of the heating wire, and k r is expressed in terms of the 

 platinum temperature scale. 



In the following tables k f is determined from the olwervations by means of this 

 equation, and the corresponding value of k n in terms of the hydrogen temperature 

 scale calculated by the equation, p. 452, on the assumption that (,'ALLENDAR'S 8 is 



constant. 



* CALLRNDAR, 'Phil Mag., 1 47, table, p. 214 (1899). 



