78 Mr. Williams on Magnetic Change of Resistance in Iron, 



to 34 c.G.s. units. In a previous paper communicated to 

 the Philosophical Magazine * the present writer gave an 

 account of measurements of the change of resistance in iron 

 and nickel at the temperatures 0° and 100° C. for fields up 

 to 800 units. 



In the experiments described below the range of tempera- 

 ture has been extended up to 665° C, and the change of 

 resistance in iron and nickel has been measured at tempera- 

 tures within this range for fields up to 900 units. 



This range of temperature includes the " critical point " 

 of nickel (380° C), and some very interesting results have 

 been obtained at temperatures near this point. In particular 

 it has been found that for some temperatures the change of 

 resistance is negative at high fields. 



Through the courtesy of Dr. C. E. Guillaume I have also 

 been able to measure the change of resistance in various 

 specimens of nickel-steel, and the results obtained are given 

 below r . 



Apparatus. 



The magnetic field was obtained by means of a coil, one 

 metre long, wound with eight layers of no. 14 copper wire. 

 This coil gave a field of 40 units per ampere^ the maximum 

 current used being 23 amperes. The heating was effected 

 by means of the electric current. A coil of platinoid wire 

 was wound non-inductively on a hard glass tube, and covered 

 with two or three layers of asbestos paper, and the desired 

 temperature was obtained by passing a current through this 

 coil. The wires whose changes of resistance were to be 

 measured were placed inside this glass tube, which was then 

 fitted inside the magnetizing coil. The coil w r as provided 

 with a water-jacket through which a current of cold water 

 was kept flowing in order to prevent the heat reaching the 

 wire of the coil. 



The temperature was measured by means of a platinum 

 thermometer placed alongside the iron or nickel wire. The 

 wires were hard-soldered to copper leads and bent double so 

 as to bring the two junctions close together and thus avoid 

 any thermoelectric effect. The platinum wire was provided 

 with double leads in the usual way, and the leads to the iron 

 or nickel wire were made of the same gauge of wire, so 

 that the effect of temperature on their resistance could be 

 allowed for. 



The change of resistance was measured by means of a slide- 

 Phil. Maj?. Dec. 1903. 



