JAN. 19, 1923 WHITE: ELECTRIC HEATING OF CALORIMETERS 25 
II. ERRORS CONNECTED WITH THE LEADS 
The proper place to put the potential terminals, whose location 
determines the amount of heat that comes into the measurement, is not 
at the ends of the constantan wire,’ but outside the calorimeter alto- 
gether, at a point such that the heat generated in the lead wire between 
it and the calorimeter equals that flowing from the wire to the calo- 
rimeter. In general, the accurate location of this point is rather diffi- 
cult, but it becomes easy if the heat given to the air can be neglected, 
since the proper point is then simply the middle of the free portion of 
the leads. The heat given to the air is easily found to be: 
QX — = tanh p X (8) 
where Q is the heat generated in each centimeter of the wire, 2X is 
the length of each lead between surfaces, and the parameter » = 
a , where F is emissivity, K thermal conductivity, A area, P 
perimeter. Langmuir’s stationary surface layer rule has been used 
to get H.8 Relatively crude data are here sufficient, since the attempt 
is only to determine when certain quantities are negligible, not to 
compute appreciable corrections. 
One ampere at 110 volts generates 4733 calories in 3 minutes. In 
1 em. of No. 24 lead wire (about 0.5 mm. in diameter) it would generate 
0.03626 calories, which number can conveniently be doubled, since we 
are dealing with a pair of leads, so the heat is 0.073 calories per cm. of 
lead length. For this wire, », calculated as in the paper just referred 
to, may be taken as 0.28, and 0.51 for No. 30 wire, while 0.16 was given 
for No. 18. We then have the values in table 1 for heat given to the 
air and not delivered at the ends, in 3 minutes. 
It thus appears that, when the voltage across coil terminals is 110, 
and the current 1 ampere, with No. 30 leads 4 cm. long, No. 24 leads 
10 cm. long, and No. 18 leads 28 cm. long, the heat lost to the air from 
the leads may be simply neglected. This is true even if the potential 
leads increase the loss to the air, which they will scarcely doif wound 
helically on the others and cemented there by shellacking. More- 
over, the heat that goes into the air will in part return to the calo- 
7Cf. H. C. Dickinson, Combustion calorimetry and the heats of combustion of 
. . Bull. Bur. Standards 11: 222, 1914. 
5 Cf, Walter P. White, The conditions of calorimetric precision. J. Am. Chem. 
Soc. 40, 1882, 1918. 
