1902.] on the Electric Conductivity of Iron and Nickel. 31 



ance divided by the resistance in zero field ; and a is a constant. It is 

 therefore suggested by Goldhammer that the above relation may be 

 also true for the ferro-magnetic metals. In order to verify this a 

 direct determination of the magnetisation is necessary. Beattie has 

 investigated the transverse effect in thin films of iron, nickel, and 

 cobalt, and determined the magnetisation by means of Kundt's result 

 that for such films the Hall effect and the magnetisation are propor- 

 tional. The relation A<£ = al 2 was confirmed in the case of the cobalt 

 films, but not in those of nickel. In iron the variation of resistance 

 was so small that accurate results were not obtained. In the more 

 recent investigation of Gray and Jones on the change of resistance of 

 iron wire magnetised longitudinally, the relation A<£ = &I 4 was sug- 

 gested as approximately representing their results between the field 

 strengths 30 and 250 c.g.s. 



The present investigation had for its object a more exact determi- 

 nation of the relation between the change of resistance and the mag- 

 netisation in the two ferro-magnetic metals — iron and nickel. As in 

 the experiments of Gray and Jones the metal was in the form of a 

 wire, so that a close approximation to uniform longitudinal magnetisa- 

 tion could be obtained. In the course of the investigation the 

 hysteresis of resistance was found to be of such importance that this 

 phenomenon was separately examined. Some determinations of the 

 transverse effect were also included. 



Experiments in Ordinary Magnetic Fields. 



These experiments were made with three different specimens of 

 wire, iron, steel, and nickel. 



The magnetic field was obtained by means of a solenoid 1 metre in 

 length, which was placed vertically. The current for this solenoid was 

 produced by a battery of storage cells, and measured by means of a 

 Kelvin graded galvanometer which was standardised from time to time. 

 The maximum field was 450 c.g.s. 



The Wheatstone-bridge method was used to determine the change 

 of resistance, the connections being shown in fig. 1. P is the iron or 

 nickel wire under investigation, Q a comparison coil of equal resistance. 

 A very thick but uniform wire of German-silver was used for the 

 bridge, but for measuring very small effects this was replaced by a 

 nickel-plated copper wire having a much smaller resistance per unit 

 length. A and B are two equal auxiliary coils of German-silver wire, 

 immersed in an oil bath to avoid temperature differences. A Kelvin 

 astatic galvanometer G of resistance 3*6 ohms was used in these 

 experiments. In some preliminary measurements on the resistance 

 change in the iron wire, the comparison resistance Q consisted of a 

 spiral coil of the same wire. This method was first used by Gray 



