PLATINUM SURFACE AT HIGH TEMPERATURES. 
243 
Emissivity in Air. 
Gas only. 
Convection diminished by filling the enclosure 
with lightly packed glass wool. 
1 rr. 
Heat 
Heat 
Heat dissipated 
Heat 
I emperat/ure 
in 
degrees 
dissipated by 
dissipated 
by conduc- 
dissipated 
Ratio 
conduction 
Emisswity. 
by 
tion of the glass 
Emissivity. 
by 
C 
and 
convection 
and air and 
convection 
cr 
ObiiLi^TcicIb. 
radiation. 
= C. 
by radiation. 
= Cl. 
1 g 
rioo 
0-00045 
0-00084 
0-00039 
0-00056 
0-00071 
0-00015 
2-60 
200 
0-00054 
0-00095 
0-00041 
0-00065 
0-00082 
0-00017 
2-41 
300 
0-00064 
0-00106 
0-00042 
0-00075 
0-00093 
0-00018 
2-33 
p 
r-H 
[400 
0-00075 
0-00119 
0-00044 
0-00086 
0-00105 
0-00019 
2-32 
§ 
flOO 
0-00474 
0-00429 
0-00176 
0-00120 
3-58 
200 
— 
0-00513 
0-00459 
— 
0-00217 
0-00152 
3-02 
El 
300 
— 
0-00553 
0-00489 
— 
0-00235 
0-00160 
3-06 
A 
o 
[too 
— 
0-00593 
0-00518 
— 
0-00247 
0-00161 
3-22 
2 
rioo 
0-00761 
0-00716 
0-00462 
0-00406 
1-76 
200 
— 
0-00808 
0-00754 
— 
0-00544 
0-00479 
1-57 
^ 1 
300 
— 
0-00856 
0-00792 
— 
0-00580 
0-00505 
1-57 
12 
^400 
— 
0-00904 
0-00829 
• - 
0 - 00605 
0-00519 
1-60 
As may be seen in the above table the result was to reduce convection to one-lialf 
or one-third of its former value. The effect on the total heat lost is of course least 
at low pressures, at which conduction plays the most important part; the decrease 
is about 14 per cent, at 1 atmosj^here, and twice or three times as mucli at higher 
pressures. 
On the Influence of Exi^erimental Conditions. 
The remaining factors which influence the total amount of heat dissi})ated are the 
dimensions of the enclosure and radiator and the temperature of the gas.'^ 
Let us first consider the effect of a change in the diameter of the radiator. 
We have found above (p. 240) that the heat lost by conduction is 
TA 
K 
(v.), 
(log, R - log.?') 
where R is the radius of tlie enclosure, r the radius of the radiator, and K the 
conductivity of the gas. 
If the ratio - is very great we may, witliin a limited ranne, consider - ^ 
r ^ logjl-log.?’ 
as constant and put 
* Some experiments on this question were recorded in Part I., hiit the following additional considera¬ 
tions may be found of interest. 
2 I 2 
