AND LIQUIDS AND THEIR VARIATION WITH TEMPERATURE, 407 



H = K -' l -BT-r ' 



where K is a constant. 



In one of the standardisation experiments the Clark cell had a temperature of 

 19 C. and an E.M.F. of 1-428 volts. The deflection on the wattmeter was found 

 to be 245'5. Hence, since R = 2'018, r = 332 ohms, we have 



1 334 



When, therefore, the wattmeter was connected to the heating coil of the discs, which 

 had a resistance of 3 '3 ohms, 



990 



H = -000986 - . d = -000976 . d, 



ooo'o 



from which the heat generated in the coil could be found from the wattmeter 

 deflection. 



The power thus measured is that spent in the heating coil and the connections 

 leading to it. A portion only of this power is imparted to the discs themselves, the 

 rest is spent in the short lengths (1'6 centim.) of the platinoid wire projecting 

 outside the discs, and in the No. 22 copper wires leading to the mercury cups outside 

 the air-bath. The total length of the platinoid wire was 150 centims., and 

 3 '2 centims. of it, and 50 centims. of No, 22 copper wire, were outside the discs. 

 The resistance of the coil and connection was 3'3 ohms, "02 ohms being due to the 

 copper, and '07 ohms to the projecting platinoid wire. Of the total power generated, 

 therefore, only 321/330 = '973 is imparted directly to the discs. 



There is, however, an indirect method by which some of the remaining power may 

 be communicated to the discs, i.e., by thermal conduction along the coil wire, and to 

 determine the amount of this, we require to know the distribution of temperature 

 along the wire. 



Assuming that the isothermal surfaces in the wire are planes perpendicular to the 

 axis, we have for the " steady state " 



7 (Pv -, 



qk d& + w = P h v > 

 where 



2 = cross section of wire, 

 p = perimeter of wire, 

 k = conductivity of wire, 

 h = emissivity of wire, 



w = heat developed in 1 centim. length of wire, 



v = temperature excess over air, at a point distant x from the junction of the 

 platinoid and copper wires. 



