406 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



Hence the formulse, p. 403, become 



1-395 + VK+V V 

 4 4 



1-395 + -:- 



"442 + --. % 



' 5 " D " 



H 



1-395 + 4- 



l-233t; + '442 + . 



1 * 

 12-5 



Measurement of Power. 



The power given to the heating coil of the discs was measured by means of a 

 wattmeter, with its series coils in the main circuit, and the terminals of its shunt 

 coil dipping into the same mercury cups as the terminals of the heating coil. The 

 shunt coil had a resistance of 12 ohms, and was suspended binlarly by means of the 

 two leading wires. The instrument was standardised by sending a current through 

 a 2-ohm resistance coil of thick platinoid wire, and regulating the strength of this 

 current till the electromotive force at the terminals of the resistance was equal to 

 that of a Clark cell. The wattmeter readings were taken by reversing the con- 

 nections of the suspended coil to the heating coil in which power was to be measured. 

 The series coils were placed parallel to the magnetic meridian, so that the effect of 

 the earth field on the deflection would be small, and there would be no necessity for 

 reversing the current in the heating circuit. For convenience in working out results 

 approximately, the resistance in series with the suspended coil (320 ohms) was 

 adjusted till the wattmeter deflection was nearly 1000 divisions per gram degreet of 

 heat, generated per second in the heating coil. 



If H is the heat generated per second in a coil of resistance li, and d is the 

 deflection on the wattmeter, the shunt coil of which has a resistance of r ohms, we 

 have 



f Tho value of h is only required in this work to make certain small corrections in the course of the 

 calculation, and the above equation will not be further referred to. The mean values of h found during 

 the course of the work, are : 



t Water gram degree at 15 C. = 4'20 joules. 



