124 Mr. E. A. Owen on Change of Resistance of Nickel 



the gaps R and S. These two coils were immersed in the 

 same oil-bath. Brass springs, attached to two ebonite riders 

 Ri and R 2 (fig. 2), were always in contact with the bridge- 

 wire. These springs were connected to mercury keys which 

 could be worked from a distance so as not to bring the hands 



Fiff. 2. 



T : Terminals attached to 

 pensathig coil. II, S : 

 M : Mercurv keys. 



bridge-table to receive cable. G : Corn- 

 Auxiliary coils. B : Broca Galvanometer. 



near the bridge connexions. In the inner gaps, E and F, 

 of the bridge were placed respectively the specimen examined 

 and a very thin platinum wire used as a comparison resistance. 

 These were mounted on two carriers, and placed in the same 

 trough which had pure paraffin oil flowing slowly through it 

 (see fig. 3 a). They were placed close together so as to be 

 subjected, as nearly as possible, to the same external changes 

 of temperature. In series with the specimen examined was 

 placed a compensating coil of platinoid wire, whose resistance 

 was adjusted so that the temperature coefficient was the 

 same for the two branches E and F. 



If G is the resistance of the compensating coil, then its 



value is given by the formula G = N . - — -, where N is the 



resistance of the specimen ; a and j3 the temperature 

 coefficient of platinum and the specimen respectively. 



A number of different suspended coil galvanometers were 

 tried, but not one was found sensitive enough for the 

 experiment. The galvanometer ultimately used was a Broca 

 galvanometer with coils whose combined resistance was 

 about 90 ohms. This instrument was found to be quite 

 sensitive enough and it worked exceedingly well. It was 

 used with a telescope and scale, the lamps used to light up 

 the scale being immersed in a glass trough through which a 



