HALL. — CONDUCTIVITY OF SOFT IRON. 



131 



the labor and observations of hours. The platinum thermometer method 

 is not without ditficulties, as the following pages will show ; but it ap- 

 pears preferable to the other. 



Figure 2 represents one of the platinum spirals, S, in position for use. 

 It consists of about 22 cm. of wire, 0.012 cm. in diameter. The diameter 

 of the coils is about 0.45 cm. The ends of this platinum wire are soldered 



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Figure 2. 



to copper wires, C and C, each about 1.6 m. long and 0.1 cm. in diameter, 

 which extend through the hard rubber plug R. Each of the copper 

 wires is soldered at the outer end to a copper rod 5 era. long and 0.6 cm. 

 in diameter, which is well amalgamated at the free end and serves to 

 make connection with a mercury well of a Carey Foster bridge. Care was 

 taken to make the length of the wire very nearly equal in the two spirals, 

 and equal care to make the copper wires, all of which are from the same 

 piece, all alike at first. The parts of Figure 2 which are in solid black 

 represent metal ; the parts cross-liatched thus / represent soft rubber. 



The resistance of each spiral was about 1.4 ohms, and that of its con- 

 necting copper wires about 0.1 ohm. Trial showed that one spiral with 

 its connecting wires had a resistance about 0.0006 ohm greater than that 

 of the other spiral and its connections. Slight changes in the copper 

 wires reduced this difference of resistance to something like 0.000025 

 ohm. As the original difference of resistance was probably nearly all 

 in the spirals, these continued to differ, at the temperature of the room, 

 by something like 0.0006 ohm, which would correspond to about one- 

 seventh part of a degree difference of temperature. No attempt at a 



