SS —— 
ON MR. SIEMENS’S PYROMETER,. ~ 245 
Combining the values (1) and (2), we get for the value of 8 in the equation 
R,=R,,[1+6(10—24)], 
b=0-002764, 
and consequently the formula for the reduction of observations to 10° becomes 
1+0:000309 (@—10) 
1+0-002764(¢—10) 
where @ is the temperature of the air inside the box of German-silver resist- 
ance-coils, ¢ the temperature of the water surrounding the pyrometer, Ryg 
the observed resistance, and R,, ,, the value which would have been found if 
both standard and pyrometer had been at 10°, 
The same correction was found also to apply to Nos, 411 and 414; but 
No. 445 appears to have been made with a different quality of platinum wire, 
for its resistance varied with changes of temperature at a perceptibly more 
rapidrate. Measurements of its resistance made at 100°5 and at 9°45 (mean 
of 9°-25 and 9°-65) gaye for the correcting factor the value b=0-00307, and 
hence the formula for correcting the measurements with this pyrometer was 
R R 1+0:000309 (6 —10) 
Bo 1-E0-00a07 . (t= 10) 
The course of testing to which each pyrometer was subjected consisted in 
heating it repeatedly to redness and determining its resistance at the atmo- 
spheric temperature after cach heating. The source of heat most often used 
was the laboratory fire in an open grate without blower; but in some of the 
later experiments a small Hofmann’s gas combustion-furnace, with three rows 
of clay burners, was employed. Rough measurements of the resistance of the 
pyrometers were made while they were in the fire in order to find out approxi- 
mately how long the temperature continued to rise, and whether it was about 
the same in the different experiments. 
It will be seen from the Tables of results which follow that, on the whole, 
the later measurements agree better with each other than those made at the 
beginning of the trials. This is no doubt to a great extent a natural result 
of practice in the use of the methods*, but it is also probably due in part to 
the greater sensibility of the galvanometer employed in the more recent ex- 
periments. The galvanometer used at first was a thin wire double-needle 
galvanometer by Watkins and Hill, of about 136 ohms resistance; this neces- 
sitated the use of a comparatively powerful testing-battery (three cells of 
Marié-Davy, zinc, carbon, sulphate of mercury), and it was consequently not 
always easy to prevent the resistance of the pyrometer being changed by the 
testing current. In all the recent experiments a reflecting galvanometer of 
very low resistance, by W. Grant, has been used, and a single Smee’s cell has 
been used as the testing-battery. 
The results of the measurement of each pyrometer are here given in the 
order in which they were made, The symbols ¢, 0, Ry, R,,, ,. at the head of 
the columns have the meanings already given. G stands for the resistance of 
the German-silver coils, and E for the resistance of Elliott’s coils inserted in 
Rio, w= Rea 
10, 10 
multiple arc with G to balance the pyrometer : Gene Ree. 
* One point, which was certainly not attended to sufficiently to begin with, was the 
importance of avoiding any thermoelectric action between different parts of the circuit, 
in consequence of which, when the resistance of a pyrometer was taken within about a 
couple of hours of its being taken out of the fire, the result sometimes differed considerably 
from what was found next day. 
