394 
PROFESSOR C. H. LEES: THERMAL AND ELECTRICAL 
For the platinoid wire the values of the constants are q = 0'00012 sq. centim., 
k = CL050 (mean), p = 0‘046 centim. over silk, h — 0‘00022 (mean). 
Hence 
’ 2 = (1-67) 1/2 = 1-29, = F03, {pqkf 2 = 5*2 x lO’ 4 , and (phqkf 2 = 78 x 10~ 7 . 
4-19Lp^ = 1-31 x 10" 3 . 
For the copper wire, — 0'0033 sq. centim., Jc± — 1*00 (mean), p 1 = 0'27 centim. 
over silk. 
HgI1C6 
{pxqAf 2 = (9 x nr 4 ) 1/2 = 0 - 030 . 
The how of heat into the sleeve from the two wires per second will therefore be 
= 0-0060W —0-000020F' c ,. 
Thus of W watts spent in the wire, only 
f — +0-0060^) W — 0-000020FV, 
\30-6 ) 
t 0-954W-0‘000020z/'' c , 
reach the sleeve, where v" c , is the excess of the mean temperature of the sleeve over 
that of the surrounding tube. 
The watts were measured at the ends of the copper leads, for the resistance of which 
a further correction must be made, a correction which will depend on the temperature 
of the leads and therefore of the tube. Each lead of copper wire, (P065 centim. 
diameter, was 55 centims. long, and of this length 33 centims. was in the Dewar tube, 
22 centims. in air. The resistance per metre at the temperature of the air, 15° to 
18° C., may be taken as 0‘051 ohm, and the total resistance of the leads as 0 - 056 ohm 
at the temperature of the air. When the platinum temperature of the Dewar tube 
is r, we may, with sufficient accuracy, take the resistance of the leads to be 
(P056 (— + — ^ ohm 
\5 5 290/ 
= 0'022 + 0‘000116r. 
If, therefore, A amperes flow through the coil, and give a difference of potential of 
P volts at the terminals, the watts W spent in the platinoid wire 
= A {P —(0-022 + 0-000116t) A}. 
Hence the watts expended on the sleeve 
= 0-954A {P-(0-022 + 0-000116r) A}-0-000020ff' c ,, . . . (16) 
where v" c , is the excess of the mean temperature of the sleeve over that, r, of the tube. 
