PLATINUM SURFACE AT HIGH TEI\IPERATURES. 
233 
of the two sides of the reversing switch. They were in a large degree eliminated 
in subsequent work. As however the mean of the two readings is always taken, 
variations of this kind hardly affect the results. 
The method of working out the observations has in all cases been identical to that 
shown in Table I., but for the sake of brevity and clearness the final results have 
alone been given. 
The emissivity ex|)ressed in C.G.S. units is equal to the number of therms (water- 
gramme-degrees) dissipated, per square centimetre of surface of radiator,"^" per second, 
per degree Centigrade above the temperature of the enclosure. During the experi¬ 
ments the temperature of the enclosure was usually about 18° C. The results of the 
observations are recorded in Tables 11.-VI. In the case of each series the pressure 
indicated is the absolute pressure of the gas when no current is })assing through the 
radiating wire. The j^ressure readings recorded have been corrected for the tempera¬ 
ture of the manometer, the height of the mercury column and the departure of air 
from Boyle’s law, this latter correction being made according to Amagat’s results. 
The importance of having some data with regard to the total heat dissipated 
cannot be denied, but our knowledge of the phenomena is not at present sufficient 
for a general theory to be evolved. It was therefore thought preferable to reduce 
the results in such a manner that they should not involve any assumptions which 
later work might prove to have been unjustifiable. It must be admitted tliat the 
emissivity as above defined is dependent to a certain degree on experimental condi¬ 
tions, such as the size and shape of the radiator and enclosure. But the advantage is 
gained that, with these conditions specified, it is a well-defined quantity, and that 
when the laws which govern the transfer of heat through high-pressure gases become 
accurately known,, the generalisation of the })resent results will present no diffi¬ 
culties. The choice of the size of the radiator and enclosure was made with a view 
to minimise the objection tliat has beoi pointed out. 
Before passing to other considerations, two possible causes of error require inves¬ 
tigation. If what Smoluchowski ue SmolanI has called the discontinuity of 
temperature—the equivalent of what is known to electricians as contact resistance— 
exists between the enclosure and the contained gas, the observations will give resnlts 
too low. 
This effect, however, is inversely proportional to the pressure of the gas, and 
above 1 atmosphere would correspond to an effective increase in the radius of the 
enclosure of less than 0'00001 centim. It need not, therefore, be brought into 
consideration. 
A more serious cause of error lies in the fact that the walls of the enclosure being 
2 centims. tliick, tlie inside surface might be consideraljly above the tempera- 
* All the results are referred to the area of the radiator as measured at a temperature of about 18° C. 
t ‘Phil. Mag.’, vol. 46, p. 192, 1898; also E. Wakduko and E. Gkhrke, ‘ Aniialen der Physik.,’ 
4th s., vol. 2, p. 102, 1900. 
VOL. CXCVII.—A. 
2 H 
