ON MR, SIEMENS’S PYROMETER,. 243 
C, X, and X,, are not only taken of the same length and gauge, but are insu- 
lated with a covering of india-rubber and tape and made into a cable of 
G. Resistance-coils. b. Testing-battery. 
HE. Platinum-silver resistance- g. Galyanometer. 
coils. k, Key. 
three strands, so that they are all of them exposed to similar conditions as to 
temperature, &c. 
The most important points to ascertain, in relation to the applicability of 
Siemens’s pyrometer to the purposes for which it is intended, were how nearly 
the resistance of the coil is constant at a given temperature, and, in case of 
its being found to be permanently altered by exposure to high temperatures, 
to determine the extent of such alterations. The investigations of the Com- 
mittee have been confined to these two points. The whole of the measure- 
ments have been made in the Physical Laboratory of University College, 
London, by Professor G. C. Foster, or his assistant, Mr. W. Grant, or else by 
students working in the laboratory under Professor Foster’s supervision. The 
thanks of the Committee are specially due to Mr. Charles Law and to Mr. O. 
J. Lodge for their valuable aid thus rendered. The method adopted was that 
of the differential resistance-measurer. Two sets of resistance-coils, both of 
them adjusted to the British-Association standard, were available for the 
measurements. One of them, made of platinum-silver wire, by Messrs. Elliott 
Brothers, gave any whole number of ohms from 1 to 10,000; the other, of 
very thick German-silyer wire, by Mr. Grant, gave any whole number from 
1 to 200. The resistance of all the pyrometers was marked upon them by 
the makers as being equal to 10 Siemens’s mercury units at 0° C., and at the 
highest temperature to which any of them was exposed during the experiments, 
the resistance never rose beyond about 36 ohms; hence the resistances to be 
measured always lay within the range of the German-silver coils. In order 
to be able to estimate fractions of a unit, the following method was adopted :— 
after the smallest whole number whose resistance exceeded that of the pyro- 
meter had been found on the German-silver coils, the other set (Elliott’s) 
were connected in parallel circuit with these, so as to act as a “shunt,” and 
R2 
