THERMAL EMISSIVITY OF THIN WIRES IN AIR. 
397 
IY. Comparison of the Results with those obtained by MM. Dulong and Petit. 
All the emissivity carves in figs. 11 and 12 (Plate 14) have the same general 
shape, being concave to the axis of temperature, and it is interesting to see how far 
this general shape agrees with the results obtained in the classical researches of 
MM. Dulong and Petit for the loss of heat from thermometer bulbs in air. 
The formulae they developed for the loss of heat by radiation and convection in air 
lead to the following expression for the emissivity :— 
e = - {Ha e (cd - 1) + Kp 0 ' 4 ^ 1 ’ 2333 }, 
where 6 is the temperature of the enclosure, t the excess temperature of the cooling 
body, a a constant having the value 1"0077 if the temperature be measured in degrees 
Centigrade, p the pressure of the gas, H a constant depending on the nature of the 
surface of the cooling body and of the enclosure, and K a constant depending mainly 
on the nature of the gas surrounding the cooling body and but very slightly on the 
nature of the surface of the cooling body. 
Substituting the value of a and expanding in powers of t we have 
- e = 0-0076705 X POO77 0 .H.{1 + 0-00383526 t + 0-000009806 t 2 
+ 0-0000000188 $ + 0-0000000000029 C + . . .} 
+ K._p°' 45 .C 333 . 
If y be used to stand for the expression in the brackets and y' for £°'- 33 , calculation 
shows that y and y' have the following values for the different values of t :— 
Values of 
t. 
y ■ 
y'- 
0 
l 
0 
10 
1'03935 
1-7100 
100 
1-50067 
2-9242 
200 
2-31430 
3-4367 
300 
3-56405 
3-7772 
And using for H and K the values found by Mr. Hopkins for polished limestone 
cooling in air at 760 millims. pressure, contained in an enclosure at 0° C., the 
expression given above for e becomes 
