Magnetic Change of Length and Resistance in Nickel. 431 



care was taken to prevent them touching each other, as this 

 was found to impede the motion o£ the nickel wire, and to 

 affect the accuracy of the measurement of the change of 

 length. 



The change of length was measured bv the well-known 

 lever and mirror method, the short arm of the lever pressing 

 upwards against a projecting piece attached to the upper end 

 of the nickel wire. The lever ratio was 15, and the total 

 magnification 18,000. The maximum deflexion obtained on 

 the scale was about 50 cms., which could be easily read to a 

 millimetre. 



The change of resistance was measured by means of a 

 slide-wire bridge. The bridge-wire was of german-silver. 

 2*75 mm. diameter, with a resistance of '000361 ohm per cm. 



The resistance of the nickel wire was 2*5 ohms, and that of 

 the copper wire used for comparison was 2*6 ohms. The 

 nickel and copper wires were connected in the outer gaps of 

 the bridge, and two auxiliary coils * of german-silver wire 

 of about the same resistance were connected in the inner 

 gaps. These were placed together in an oil-bath in order to 

 maintain them at the same temperature. The connexions 

 were made by thick copper rods, soldered to the brass ter- 

 minals of the wires, except in the case of the free end of the 

 nickel wire, where a short piece of flexible wire was used. 



The change of resistance in nickel at the highest fields 

 used is about one per cent., and this corresponds to a step of 

 40 cms. on the bridge. 



The ends of the nickel wire being at opposite ends of the 

 magnetizing coil the thermoelectric E.M.F. was generally 

 very considerable. It is easily seen, however, that it does 

 not produce an appreciable error in the measured change of 

 resistance (owing to the relatively small value of the effect) 

 provided that the E.M.F. is constant during the time of 

 taking the two readings. In order to secure this it was 

 found necessary to allow the current to flow constantly through 

 the bridge circuit. 



The error arising from this cause, and also from the heating- 

 effect of the magnetizing current, was further diminished bv 

 taking the readings rapidly. In order to facilitate this, two 

 sliders were used on the bridge, connected to the galvanometer 

 by a double key, so that either slider could be used at will. 

 One slider was used to balance when the magnetizing current 

 was on, and the other when it was off. In this way it was 

 found possible to arrange so that the zero did not change 

 appreciably during the time of taking a reading. 



* Cf. Gray and Taylor Jones, Proc. Hoy. Soc. vol. lxvii. p. '208. 



