BETWEEN THE FREEZING AND BOILING-POINTS. 
183 
pt = 
T> _ p 
-^100 ■ Lt o 
x 100, 
where R 0 , R 100 and R/ are the measures of the resistance of any one particular sample 
of platinum wire at 0°, 100°, or at a temperature t, has been now almost universally 
accepted. 
The reduction of the platinum temperature to the air-scale was obtained from a 
series of comparisons with the nitrogen air thermometer at three fixed points 0°, 100° 
and 444°, which led to the well-known parabolic formula, 
Too 72 “ too)’ 
where t is the air temperature, and § a constant depending on the purity of the 
platinum wire, the same for any particular purity of wire. 
In selecting the wire for use in the present measurements, I was exceedingly 
fortunate in possessing a sample of the original wire standardized by Professor 
Callendar and Mr. Griffiths, who found its S equal to l - 50. 
The chief difficulty in selecting a form of thermometer for use in the calorimeter lay 
in choosing a size of bulb which would give a sufficiently large change in resistance for 
the rise of temperature produced in the water. On a rise of temperature of 10 c , it 
was necessary to be sure of the measurement to '001°, and to obtain the readings to 
•0001° to have them comparable with the accuracy of the other measurements. At 
the same time it was impossible to have the bulbs too long, as it introduced 
increased possibilities of error in the outflow-tube of the calorimeter. For the size 
of wire used (T5 millim.), and the size of the units in the resistance-box for 
compensating the change in resistance, it was necessary to use about 4 metres of wire 
for each thermometer. 
The first thermometers made were from some of the original sample of wire, which 
had been silk covered. Four metres of this wire were coiled up into a bulb, about 
6 centims. long, and half a centim. in diameter, which served the purpose very well. 
Two sets of thermometers were made this way at different times, and will be 
described further on. The chief difficulty with this form was that, after bending 
into the coil, the wire could not be annealed well enough. Annealing for a length of 
time at 150° C. served to give fairly steady results. The difficulty, caused by the 
exciting current, of heating in the interior of the coiled wire, was also a serious 
question, which had to be carefully considered. 
A pair of thermometers was made for the first tests with the water calorimeter, 
which were in the usual form of bare-wire wound on a mica frame. To keep the 
length of bulb within reasonable bounds, it was necessary to have these thermometers 
only one-half as sensitive as the others. However, this form was far preferable to 
the other, so that to produce the same sensitiveness as was required, with the most 
