Nickel, and Sickel- Steel at Various Temperatures. 79 



wire bridge. In order to obtain a large step on the bridge 

 a thick copper wire was used, and two sliding keys were 

 made with the connexions entirely of copper, so as to avoid 

 any thermoelectric effects in the galvanometer circuit. One 

 of these keys was used to balance when the magnetic field 

 was " off " and the other when it was " on." By the use of 

 two keys in this way the comparison of resistances can be 

 carried out in a few seconds, which it is necessary to do as 

 the resistance in zero field is continually changing owing to 

 slight changes in the temperature of the wire. The iron or 

 nickel wire under investigation was connected to one of the 

 inner gaps of the bridge, an adjustable resistance being- 

 placed in the other, and two auxiliary coils of about 5 ohms 

 each were placed in the outer gaps of the bridge. This 

 arrangement has the advantage that the constant for reducing 



the brid^e-readinos to the form ^ , R being the resistance 



of the wire, is independent of R to a first order of approxima- 

 tion, and it is not, therefore, necessary to calibrate the bridge 

 separately for different resistances of the wire. 



The arrangements described above worked very satis- 

 factorily for nickel and iron up to a temperature of about 

 500° C. The change of resistance in nickel vanishes at about 

 400° C, but the change in iron persists to a much higher 

 temperature. 



In attempting to measure the change of resistance in iron 

 at these temperatures several difficulties were met with, and 

 a slightly different arrangement of apparatus was adopted. 

 In order to have space for a thick covering of asbestos around 

 the heating coil, a large magnetizing coil was used having an 

 internal diameter of 6 cms. The heating coil was of nickel 

 and was wound on a porcelain tube and covered with asbestos 

 powder. The nickel wire is non-magnetic at temperatures 

 above 400° C, and therefore does not interfere with the 

 magnetic field inside the coil. 



At temperatures above 500° C. the iron wire if in contact 

 with air oxidizes rapidly, and hence the resistance of the 

 wire continually increases. For this reason the wire must 

 be heated either in nitrogen or in a vacuum, the use of 

 hydrogen being debarred owing to its action on the platinum. 

 The arrangement adopted for this purpose is shown in fig. 1. 

 The heating-coil was placed inside a glass tube which was 

 about a foot longer than the coil. The connecting-wires 

 were led out through a cork in the top of the tube and the 

 joint was made air-tight with wax. 



