186 Dr. Harrison on Variation with Temperature of 
respectively, and a is the mean temperature coefficient of 
resistance between a and 100°. 
Phe’ value ofc" a for ‘each platinum tube was obtained 
from observations of its resistance in ice and steam. For 
two of the tubes used, the coefficients were 0°00387 and 
000394: values which were sufficiently high to justify the 
use of 1°5 as the difference coefficient A in Callendar’s 
formula 
t ‘=A ( Peary ye 
rae ON 100 Mine: 
where “t¢”’ represents the temperature on the air-thermometer 
seale. 
For conversion to air-seale, curves were constructed from 
this formula by plotting “t—pt” against “pt,” and all 
corrections to air-scale are made graphically by means of 
tnese curves. 
Before beginning a series of magnetic observations, 1t was 
necessary to determine the resistance at 0° of that portion of 
the platinum tube which lay between the two side leads 
(Ry in formula 1). When the heating apparatus was in 
position inside the solenoid, the resistance of the platinum at 
the temperature of the laboratory was determined, and from 
this was deduced the value of its resistance at 0°. 
Method of taking the Observations. 
The method adopted was first to raise the temperature of 
the specimen to a definite value, and while this value was 
kept as steady as possible, to vary the magnetizing force. 
{n the first set of experiments with nickel, a vacuum was 
not necessary, and the operations were as follows :—the 
nickel having been demagnetized by reversals and removed 
from the solenoid, the compensating coil U, (fig. 1) was 
adjusted until a current in the solenoid ceased to affect the 
magnetometer. The heating current was then allowed to 
run for 10 minutes, when observations of platinum resistance 
were made, and repeated at intervals of 3 or 4 minutes. As 
soon as a steady temperature was attained, the magnetizing 
current was started, and increased step by step, the corre- 
sponding magnetic deflexions being noted. The nickel was 
then taken through several complete cycles, during the last 
of which were recorded the values of current and magnetic 
deflexion. Finally, another set of resistance measurements 
were taken. 
The temperature was calculated from the mean value of 
the resistance before and after the magnetic observations. 
