Change of Resistance in Iron produced ~by Magnetisation. 209 



were used, of nearly equal lengths, cut from the same specimen, and 

 doubly wound with silk. One of these was wound longitudinally, the 

 other spirally and non-inductively, on a rod of wood about 60 cm. 

 long. The whole was placed inside a magnetising coil, so that one 

 coil, P, became longitudinally, the other, Q, transversely magnetised. 



On account of the great demagnetising factor for a cylinder mag- 

 netised transversely, however, the latter coil was only very feebly 

 magnetised. The magnetising coil was 1 metre long, and was pro- 

 vided with a double cylindrical core through which a constant stream 

 of water could be kept flowing, in order to diminish the heating 

 effect of the magnetising current. It was further arranged in the 

 experiments that equal currents should flow in the two iron coils, 

 which were thus subjected to very approximately equal rises of tem- 

 perature. These arrangements, though not perfect, so much di- 

 minished and retarded the heating effects of currents in the various 

 coils that these could be readily distinguished from the magnetic 

 effect. 



The comparison of the resistances of the coils P, Q was carried out 

 in the usual way. The platinoid wire of the bridge was replaced by 

 one of somewhat thick copper, so as to give an easily measurable step 

 of the slider for the small alteration of the ratio of resistances of the 

 coils which had to be measured, the iron coils inserted in^>, q (fig. 1), 

 and two nearly equal coils A, B in a, b. A and B were both kept in 

 one bath of oil. All connecting wires were of thick copper. The 

 galvanometer was a Kelvin low-resistance astatic instrument. The 

 magnetising current was measured by a Kelvin graded galvanometer, 

 standardised by electrolysis and by comparison with a Kelvin deci- 

 ampere balance. 



In making observations, the iron coils P, Q were first demagnetised 

 by reversals, and the position of the sliding contact key S found, which 

 gave no galvanometer deflection. 



Then a weak magnetising current was applied, reversed several 

 times, and the position of S again found. The change of position of S 

 indicated that the resistance of the longitudinally magnetised wire 

 became greater than that of the transversely magnetised wire. This 

 was repeated for a large number of field-strengths, each greater than 

 the one preceding it. The resistances of the auxiliary coils A, B were 

 so chosen that the change of position of S with the greatest mag- 

 netising field used amounted to about 16 cms. The difference of the 



/AP AQ 

 fractional increments of resistance of P and Q ( -^- - -^- = A< j was 



\ * Vs / 



calculated from the observed displacement of S from its zero position, 

 and the results given below represent values of 8<f> for different field- 

 strengths. 



