BETWEEN THE FREEZING AND BOILING-POINTS. 
189 
were removed, the contacts of each corresponding compensating coil were removed at 
the same time. To obtain the fundamental constant (R 100 — R 0 ) or interval, F.I., 
both C and P are balanced when immersed in melting ice, and then with C in ice and 
P in steam. To obtain the difference between the intervals of the two thermometers 
both are read when in steam. This gives the data required for converting into 
degrees a change of resistance in P relative to C. 
During the progress of the present experiments, five pairs of differential thermo¬ 
meters were made and tested. In describing these, I shall letter them A, B, C, D, 
and E, respectively. The thermometers of pair A were made of the original silk- 
covered •15-millim. platinum-wire, about 25 ohms resistance each. The bulb of each 
thermometer was about 6 centims. long and was fastened by solder joins to flexible 
copper leads placed side by side with compensating leads. The protecting tube was 
of glass, about 25 centims. long and a few millims. in diameter. The ends of the 
compensating leads near the bulb were connected by a small piece of platinum-wire 
about 4 centims. long. This was to correct any conduction error on the wire in the 
thermometers by heat conduction from the leads. This device was also used for all 
the other thermometers. 
Thermometer B was made of 12|~ohms resistance, or one-lialf the sensitiveness of 
the other pair. Each thermometer of the pair was wound in the usual way on a 
mica frame, from the 6-millim. bare platinum-wire, and annealed at a low red heat. 
As these thermometers proved eventually to be too bulky for convenience in the 
calorimeter, they were soon discarded. It will, therefore, not be necessary to give 
them further mention. 
Thermometer C was made from a pair of silk thermometers similar to A. The 
platinum-wire was fused to copper-wires, which in turn were soldered to copper leads. 
These thermometers proved satisfactory in many ways, although they finally gave 
trouble from defective insulation and had to be abandoned. These thermometers 
were used in our first preliminary measurements of J during the summer of 1898. 
Thermometer D was the first pair made from ‘10-millim. platinum-wire. This wire 
was some sent out to Mr. R. O. King by the Cambridge Scientific Instrument 
Company. Its 8 was given as R50, which was subsequently verified by Mr. Tory in 
the course of his work. Each thermometer was about 20 ohms, in resistance, and 
was made by winding on a mica frame. The bulb was about 5 centims. long and 
between 6 and 7 millims. in diameter. Owing to the inconveniently small F.I. of 
this pair of thermometers (about 700 units of the box instead of 1000 for 100° C.), it 
was supplanted by pair E. 
Thermometer E is by far the most important pair, as with it all the later measure¬ 
ments of the specific heat were obtained. The wire used in- making each thermometer 
of the pair was drawn down to HO millim. from the original 6-millim. platinum-wire. 
The resistance of the thermometers was about 25 ohms each, and gave a F.I. about 
970 units of the box. The bulbs were about 7 millims. in diameter and about 
