1892.] of a Conductor when transmitting a Current. 



21 



developed in a platinum wire, it must of necessity have been 

 at a higher temperature than the water to which it was supplying 

 heat and therefore its resistance at any time was not that at 

 the temperature of the water when no current was passing, but 

 something greater. 



A Wheatstone's bridge (Fig.) was constructed of which 

 the arm BC contained the calorimeter coil, the corresponding 

 arm BD had very nearly the same resistance as BG, but was 

 formed of two large German-silver coils, one, belonginaf to the 

 Cavendish Laboratory, was a triple-strand, containing 1400 ft. of 

 single wire, the other a double strand of stouter wire containing 

 480 ft. ; the mass of metal in this arm was very great, its cooling 

 surface was large, and its temperature coefficient was small, so 

 that the increase in resistance in this arm for currents up to 

 1 ampere although it could be detected was scarcely measurable : 

 since these arms BC, BD were equal, the other pair AC, AD 

 were also equal, these were of the same German-silver wire as 

 the two-strand coil and were each about 5 feet long, and had 

 a resistance of '3 ohms. They were arranged as near together 

 as possible so as to be equally influenced by air-currents, &c. 



The calorimeter coil was furnished with a pair of electrodes 

 at each end, one of each pair was connected with the Clark-cell 

 circuit, the other two being connected directly at B, and through 

 the arm CA at C to the battery circuit in which was placed a 

 rheochord K by the adjustment of which we were able to keep 

 the galvanometer Q^ always at its zero, and therefore the differ- 

 ence of potential at the ends of the coil could be made that 

 of 1, 2, or more Clark cells. 



[As a full description of the rheochord, and of this "potential 

 balance," as it may be termed, will be described in a paper upon 

 which we are now engaged, we do not think it necessary to 

 inflict any further details on the members of this Society.] 



A preliminary rough adjustment of the bridge was made at 

 A, so that the bridge was in equilibrium at some temperature 



