386 
MESSRS. W. E. AYRTON AND H. KILGOUR ON THE 
number of calories (gramme C.) lost per second per square centimetre of surface per 
1° C. excess temperature, then 
0239 AY 
e 7 r. I. d (t — t 0 ) 
l and d are not, however, absolute constants for a given wire, since as the wire gets 
hotter and hotter its length and diameter become larger and larger. If y be the 
coefficient of linear increase of platinum per 1° C., which, according to Kohlrausch, 
is 0'000009, then allowing for the lengthening and thickening of the wire as it 
increases in temperature, we have, if l and d be the values at 15° C.— 
_ (1 + 15y) 3 < 0-239 AY 
e ~ (1 + y 0 3 TT.l.d{t — t 0 ) 
_ 0-076094 AY 
l.d (1 + (t — 0 )) 
Now (l -j- y^) 3 ( t — t 0 ) may be regarded as equal to ( t — t 0 ) + A (t — t 0 ) where 
A (t — t 0 ) is a correction that must be added to the observed excess temperature 
(t — t 0 ), the amount of this correction depending on the value of t. The values of 
(1 + y£) 3 (t — £ 0 ) and the amounts of the correction for different values of t are 
given in the following Table VI., and are shown plotted in fig. 10 (Plate 13). The 
corrections are but small, but they have nevertheless been taken into account in deter¬ 
mining the values of the emissivity given in the following Tables VII., VIII., and IX. 
Table VI. 
Tempei-ature t. 
(i + 7 o 3 0 - v>- 
o 
i 
■+* 
< 
° 0. 
15 
0-500065 
0-00014 
30 
15-5084 
0-008 
50 
35-532 
0-032 
70 
55-57 
0-07 
85 
70-61 
Oil 
100 
85-65 
0-15 
120 
105-73 
0-23 
140 
125-82 
0-32 
160 
145-92 
0-42 
180 
166-04 
0-54 
200 
186-17 
0-67 
240 
226-48 
0-98 
280 
266-84 
1-34 
300 
287-04 
1-54 
