394 
MESSRS. W. E. AYRTON AND H. KILGOUR ON THE 
Table XI.—Wire at 100° C. 
Diameter of wire. 
Actual 
emissivity. 
Calculated 
emissivity. 
Difference. 
Percentage 
difference. 
mils. 
1-2 
millims. 
0-031 
0-01085 
0-01104 
+ 0-00019 
+ 1-8 
2-0 
0-051 
0-00790 
0-00707 
- 0-00083 
- 11-7 
2-9 
0-074 
0-00455 
0-00520 
+ 0-00065 
+ 12-5 
6-0 
0-152 
0-00293 
0-00305 
+ 0-00012 
+ 3-9 
8-1 
0-206 
0-00280 
0-00253 
- 0-00027 
- 10-9 
9-3 
0-236 
0-00230 
0 00233 
+ 0-00003 
+ 1-6 
111 
0-282 
0-00205 
0-00212 
+ 0-00007 
+ 3-3 
14-0 
0-356 
0-00189 
0-00190 
+ o-ooooi 
+ 0'5 
Table XII.—Wire at 200° C. 
Diameter of wire. 
Actual 
emissivity. 
Calculated 
emissivity. 
• 
Difference. 
Percentage 
difference. 
mils. 
1-2 
millims. 
0-031 
0-01278 
0-01303 
+ 0-00025 
+ 1-9 
2-0 
0-051 
0-00907 
0-00826 
- 0-00081 
- 9-8 
2-9 
0-074 
0-00538 
000604 
+ 0-00066 
+ 11-0 
6-0 
0-152 
0-00346 
0-00349 
+ 0-00003 
+ 1-0 
8-1 
0-206 
0-00308 
0-00288 
- 0-00020 
- 6-9 
9-3 
0-236 
0-00259 
0-00265 
+ 0-00006 
+ 2-4 
11-1 
0-282 
0-00236 
0-G0240 
- 0-00004 
- 1-5 
140 
0-356 
0-00214 
0-00213 
- o-ooooi 
+ 0-4 
Table XIII.—Wire at 300° C. 
Diamete 
r of wire. 
Actual 
emissivity. 
Calculated 
emissivity. 
Difference. 
Percentage 
difference. 
mils. 
1-2 
millims. 
0-031 
0-01440 
0-01454 
+ 0-00014 
+ i-o 
2-0 
0-051 
0-00985 
0-00918 
- 0-00067 
- 7-3 
2-9 
0-074 
0-00584 
0-00668 
+ 0-00084 
+ 12-6 
6-0 
0-152 
0-00384 
0-00382 
- 0-00002 
- 0-5 
8-1 
0-206 
000335 
0-00312 
- 0 00023 
- 7-4 
9-3 
0-236 
0-00284 
0-00286 
+ 0-00002 
+ 0-7 
11-1 
0-282 
0-00261 
0-00258 
- 0-00003 
- 1-2 
14-0 
0-352 
0-00229 
0-00228 
- o-ooooi 
- 0-4 
The values of the emissivity given in the last table are plotted in the curve shown 
in fig. 13 (Plate 15). 
The statement not unfrequently made that the current required to maintain a wire 
of a given material at a given temperature above that of the surrounding envelope is 
proportional to the diameter of the wire raised to the power three halves, is equivalent 
to stating that the emissivity is independent of the diameter. Now if we may 
