THERMAL EMISSIVITT OP THIN WIRES IN AIR. 
403 
calculated for the 6 -mil wire for the maximum current used, viz., 1 ‘4 ampere, we 
find that 
a = 1592, 
T = 315° C. 
The temperature of the enclosure in the emissivity experiments with this wire was 
12 ° C., and as the temperature T 0 of the point of the wire where it was joined to the 
support would be somewhat higher than this, we may take T 0 as about I 5° C. It is 
to be noticed that the higher this temperature actually was the more uniform must 
have been the temperature of the whole wire and the more accurate will be our 
experiments on emissivity. 
Substituting these values for a, T and T 0 in equation ( 12 ), we have 
x= ± 3148 log f- 10-58 - 7 7 * * - > 
8 \ y/(t + 630) + ^94 b) 5 
and the following are the values of t obtained for the corresponding values of x :— 
Temperature, 
in degrees Centigrade. 
Distance in centimetres 
from the end of the wire. 
15 
0 
100 
0T23 
200 
0'339 
270 
0’646 
300 
0-980 
312 
1-498 
314 
3296 
The actual length of this 6 -mil wire between the points of support was 
29-18 centims., and the maximum temperature was 315° C. Therefore, as the 
calculated temperature is 300° C. at a distance of 0"98 centim. from the support, it 
follows that in 3*3 per cent, of the wire, reckoning from the support, the temperature 
rises to about 95 per cent, of the value it has at the middle of the wire; while at a 
distance of 1*498 centim. from the support, that is, in about 5 per cent, of the whole 
length, the temperature has reached about 99 per cent, of its value at the middle. 
The calculated distribution of temperature along this 6 -mil wire, when traversed by 
the largest current employed with this wire, is shown in fig. 16. The distance 
between the voltmeter wires was 27 "46 centims., as indicated in the figure; and it is 
with the distribution of temperature over this portion of the wire only that we have to 
deal in the emissivity experiments. Of this length, about 25"70 centims. is seen to 
be at a temperature a little over 314° C. ; while 1*76 centim. has a mean temperature 
3 F 2 
