E. L. Nichols—Flectrical Resistance, ete. 363 
. 
cient of KHaepansion of Incandescent Platinum; by 
Nicuous, Px.D. (Gottingen). 
Art. XLVIIL.—Note on the Electrical Resistance and the Coeffi- 
he 
[Read at the Cincinnati Meeting of the American Association for the Advance- 
ment of Science, August, 1881.] 
I. In the measurement of temperatures above the red heat, 
the platinum pyrometer, in one form or another, is as important 
as the mercury thermometer, at ordinary temperatures. The 
researches already completed, on the electric resistance and the 
coefficient of expansion of platinum, and on the specific heat 
of that metal, only serve, however, to remind us of the much 
that remains to be done before we may hope to attain to even 
a fair degree of accuracy in the measurement of temperatures 
above 500°. | 
The present writer in order to compare the existing formule 
for the temperature of platinum from its electric-resistance, with 
those by means of which the temperature is calculated from the 
coefficient of expansion, and thus to gain a clearer idea of the 
relative usefulness of the two methods, has determined the 
resistance and the corresponding length of a platinum wire at 
farione temperatures between 0° and the melting point of that 
metal. 
II. Upon a platinum wire 0-4™" in diameter and 100™™ long, 
determined by the following method. 
W 
circuit with the Bunsen’s bat- 
tery. A very small portion of 
the current wasshuntedaround 
ab, the portion of the wire to 
be tested, and carried through 
a sensitive sine galvanometer 
(7) and through a resistance 
coil (w) of 5000 ohms. : : 
Now with the above arrangement of apparatus, if w 1s very 
much larger than 7, the resistance of the wire ab, so that the 
