CHANGE OF ELECTRIC RESISTANCE OF NICKEL 



537 



Wheatstone Bridge circuits, which of course utterly prevented all possibility of resistance 

 measurements being made. 



The resistances of the wire x and the various branches of the Wheatstone Bridge 

 were as follows, the ohm employed being the unit of the resistance box used, that is, the 

 original British Association unit. Since the results are, strictly speaking, relative, there 

 is no need reducing to the present legal ohm. 



Branch or Wire. 



Resistance at 10° C 



M, 



2-379 



N, 



2-366 



VI, 



3-082 



V, 



3-089 



X, 



0-3015 



Galvanometer, . 



2-24 



Battery, 



5 



The magnetising current was measured on a Kelvin long range galvanometer, which 

 was calibrated in situ by comparison with a Kelvin ampere balance. 



The total length of nickel wire forming each core, excluding the ends outside the 

 anchor ring, was 459 cm. in the case of M and 458 cm. in the case of N. The total 

 number of loops in each was twenty. The total number of turns of copper wire in the 

 magnetising coil was 846 in M and 840 in N. Hence to reduce the current in amperes 

 to strength of field in the heart of the anchor ring spaces we must multiply by the factors 

 46 "4 and 46*2 respectively. 



Let us now consider the theory of the mode of experimenting and of the method of 

 deducing the final quantitative results. In the Wheatstone Bridge arrangement the 

 value of the current I through the galvanometer is given by the equation 



DI=E (Mri - rriS) 



where E is the electromotive force of the battery and D is the well-known determinant 

 involving the values of the resistances of the six conductors which make up the bridge. 

 Two of these, M and N, were subjected to various temperatures, so that the value of D 

 varied correspondingly. The changes of resistance in the M and N branches due to the 

 magnetisation were too slight to affect the value of D appreciably. At the very most 

 these changes are of the order of 2 in 500, and in the great majority of cases much less. 

 They are therefore within the errors of observation. 



When the galvanometer was gauged in the manner already described the wires were 

 at the ordinary temperature of the room. The temperature was then brought to the 

 point required and an approximate balance produced on the galvanometer. Thereafter 

 the experiment consisted in comparing the deflections produced (1) when the magnetising 

 current was applied so as to magnetise the one nickel wire, and (2) when no magnetising 

 current was passing. 



Let dl be the change of deflection due to a change dn in n produced in the opera- 

 tion by which the galvanometer is calibrated ; and let D be the value of the deter- 

 minant. Then 



