126 Drs. H. T. Brown and W. E. Wilson. [Jan. 9, 
on the revolving drum was equivalent to a temperature difference between 
the coils of 0°-2 C.; differences of 0°02 were readily measurable. 
Our experiments were originally planned with the idea of determining the 
temperature-difference between the transpiring leaves and their environment 
by a comparison of the resistances of the platinum coils when one was clothed 
with its leaves and the other was exposed to free air. We have, however, to 
consider the possible effect produced by the heating of the coils during the — 
passage of the current through them. 
Mr. Francis Darwin, in a recent paper which came under our notice at the 
time we were planning these experiments,* has described the application of the 
platinum-resistance-thermometer and the Callendar recorder to a study of 
the leaf-temperature as an index of the condition of the stomata. With the 
small platinum coils he employed, which were wound on plates of talc 
10 x 3 mm., it was found that the current used raised the temperature of the 
coils about 2° above the surrounding air, and that in order to avoid the diffi- 
culties introduced by inequalities in the rate of cooling, the central coil had 
to be covered with a “ body equalling in volume and conductivity that on the 
experimental bulb.” | 
_ In order to avoid as far as possible in our experiments the complications 
due to heating of the coils special precautions were taken in their construc- 
tion, but the heating effect was still a sensible factor when using a current of 
0°155 amperes. With a coil resistance of 13:8 chms the heating effect of the 
19 CYR . ; 
above-mentioned current in each coil was (g¢ CPR 1.¢., 0°01374 calorie per 
a p 
second, or 0°824 calorie per minute. The total surface area of a wire of 
6:006-inch gauge and 2°4 metres in length is 11491 sq. cm., so that the heat 
produced per square centimetre of surface per second is 
001374 
ARE = 0:001195 calorie. 
From the experiments of Ayrton and Kilgour on the thermal emissivity of 
bright platinum wires of 0:006-inch diameter, we know that at 20° C. the 
emissivity is about 0°00222 calorie per square centimetre per second for a 
1° excess of temperature.t Hence the maximum temperature to which the 
bare coil will be heated in “still air” by the above-mentioned current is 
0:001195 
———__ = 0°53 C. 
000222 ; 
* © Botanical Gazette,’ vol. 37, 1904, p. 81. 
+ ‘Phil. Trans.,’ A, vol. 183, 1893, p. 371. 
+ Experimentally the temperature in still air was found to be 0°54. 
