THERMAL EMISSIVITY OF THIN WIRES IN AIR. 
39 
Q 
6 
On comparing the loss of heat from the wire of 1*2 mil diameter when at 300° C. 
with that from the wire of 6 mils diameter when at 15° C., both being in an enclosure 
at 10° C., we see that the former loses per square centimetre per second, not 
300 - 10 
15 - 10 
or 58 times 
as much heat as the latter (which it would do if the emissivity were the same) but 
instead, 
60 X 58 or 3480 times 
as much heat; arising from the fact that the emissivity, that is, the number of calories 
(gramme C.°) lost per second per square centim. of surface per 1° C. excess tempera¬ 
ture of the 1*2 mil wire at 300° C., is 60 times as great as that of the 6 mils wire at 
15°, the latter varying very rapidly with the temperature near 15° C. 
From the curves on figs. 11 and 12 the following table has been drawn up giving 
the emissivities of the various wires at eight useful even temperatures. 
Table X. — Emissivities at-— 
Diameter of 
wire in 
d 
o 
O 
60° C. 
d 
O 
o 
00 
100° 0. 
150° 0. 
200° C. 
250° C. 
300° C. 
Mils. 
T2 
Millims. 
0-031 
0-00823 
0-00956 
0-01030 
0-01085 
0-01187 
0-01278 
0-01362 
0-01440 
2-0 
0-051 
0-00595 
0-00686 
0-00750 
0-00790 
0-00860 
0-00907 
0-00948 
0-00985 
2-9 
0'074 
0-00219 
0-00334 
0-00409 
0-00455 
0-00509 
0-00538 
0-00563 
0-00584 
6-0 
0-152 
0-00246 
0-00266 
0-00280 
0-00293 
0-00321 
0-00346 
0-00367 
0-00384 
8T 
0-206 
, . 
, # 
0-00280 
000294 
0-00308 
0-00322 
0-00335 
9-3 
0-236 
0-00230 
0-00245 
0-00259 
0-00272 
0 00284 
111 
0-282 
. , 
0-00205 
0-00222 
0-00236 
0-00249 
0-00261 
14-0 
0-356 
0-00189 
, 
0-00203 
0-00214 
0-00222 
0*00229 
- 
The next step taken was to express mathematically the law connecting the emis¬ 
sivity of a wire with its diameter for a fixed temperature, and this was done for three 
fixed temperatures, viz., 100° C., 200° C , and 300° O. Using the values of the 
emissivity contained in the three columns headed with these temperatures in Table X. 
we obtained:— 
For 100° C., e = 0*001036 + 0*012078 cl~ l . (5), 
„ 200° C., e = 0*001111 + 0*014303 cU 1 .(6), 
„ 300°C., e — 0-001135 + 0-016084 cl~ l .(7), 
where d is the diameter of the wire in mils or thousandths of an inch. 
The following Tables, XL, XII., and XIII., give the actual emissivities, the emis- 
sitives calculated by means of the three preceding formulae respectively, the differ¬ 
ences, and the percentage differences. 
MDCCCXCII.—A. 3 E 
