Brown — The Change in Young' 's Modulus of Nickel. 187 



The temperature of the room during the experiments was kept as uniform 

 as possible at about 17° C, and did not vary more than 0-3° C. on either side. 

 According to Walker 1 the change in the Young's Modulus per 1° C. between 

 16° C. and 20° C. is for nickel about 0-21 of one per cent.; hut the accuracy 

 of the results recorded may be taken to lie between these limits. Previous 

 to each set of observations for any given applied direct longitudinal magnetic, 

 field, the wire was demagnetised by applying a gradually decreasing alter- 

 nating current round the solenoid, the alternating current having an initial 

 maximum value equal to approximately that of the direct current about to be 

 applied. 



There was no perceptible heating of the wire when a direct longitudinal 

 field was round it, but there was slight heating when alternating fields were 

 used, and to overcome this, the following method of observation was 

 adopted: — With the constant load on the wire, and a given alternating 

 magnetic field round it, the hair in the microscope was set on the zero mark, 

 that is, the point of crossing of the spider lines; the current was then' put off 

 and the reading on the disc of the micrometer taken, the working load of 

 2 kilos, was put on the scale-pan, the field put on, and the microscope ao-ain 

 set to the mark; the field and the working load were then taken off and the 

 zero again tested with the field on. From these readings, repeated five times 

 for each separate magnetic field applied, the mean was taken as the correct 

 reading. The largest current used round the solenoid was about 2 amperes, 

 and a tap-key was employed in the circuit, so that, in every case the 

 alternating current was on not more than five seconds each time, an interval 

 of from 3 to 5 minutes being allowed between each reading in the set of five. 

 An interval of about 10 minutes was allowed to elapse between the application 

 of each value of magnetic field. It may be assumed, therefore, that any error 

 due to the heating of the wire by the alternating field was very small and 

 did not affect the results within the working limits of the investigation. 



The nickel wire employed in the experiment, as stated above, was 

 226 cms. long and 01685 cm. in diameter, of simple rigidity, about 

 715 x 10' grammes per sq. centimetre. The constant load on the wire 

 was 10 5 grammes per sq. cm., and the working load 2 kilos. The wire 

 was put through three sets of experiments, namely, when it was under 

 the influence of direct longitudinal magnetic fields, and when under the 

 influence of alternating longitudinal magnetic fields of frequencies 25 and 

 50 per second respectively. 



The results are given in Table I, and shown in the form of curves in 



1 Proc. Roy. Soc. Edin., vol. xxxi. 



■I H -A 



