produced in Iron Wires by Magnetization,. 353 



The core of this coil was made of two co-^axal brass cylinders 

 fastened together by the end plates of the coil. The space 

 between these two cylinders was filled with water, which 

 proved to be an excellent way of retarding the heat-effects 

 produced by the current. 



Method. 



I at first intended to measure the elongation and the corre- 

 sponding induction simultaneously, but found this difficult, 

 and so adopted a more convenient and apparently as accurate 

 a method. This was to first measure the current used to 

 produce a given intensity of magnetism and the consequent 

 changes in length, and afterwards to get the relation between 

 the strength of field and the intensity of magnetism in the 

 iron, the apparatus in the meanwhile remaining untouched. 

 As the laboratory is situated in the city, the work had to be 

 done late at night after traffic had stopped, for, in spite of all 

 the precautions that could be thought of, the shaking of the 

 apparatus could not be prevented. For this reason the induc- 

 tion was not determined immediately after obtaining the 

 elongation-curve, but was done the next morning. The 

 induction was measured by the method of reversals*. The 

 induction-coil used for this purpose consisted of 200 turns of 

 number 36 wire wound in one layer on a paper cylinder 

 slightly greater in diameter than the specimen. This little 

 cylinder was slipped over the wire and fixed halfway between 

 a m. The galvanometer was calibrated by means of a standard 

 coil having a wooden core. The secondary of the standard 

 coil also consisted of 200 turns of wire, which plan saved much 

 computation. The intensity was then calculated by the 

 formula r> = H + 47rI. Knowing the change in length and 

 the intensity of magnetization for any current, of course the 

 relation between these two quantities could be easily plotted. 

 The current was supplied by a battery of storage-cells, and 

 the resistance was regulated by a slide-resistance of copper- 

 sulphate. 



Results. 



My first results were obtained from a specimen of mode- 

 rately soft commercial iron wire, 1 millim. in diameter. This 

 wire was free from stress except that due to the jacket-cylinder 

 screwed to it, which weighed 350 grams. The elongations 

 given (Table I.) are those due to temporary magnetism, and 

 each value is the mean of two or three readings which did not 



* Ewing, London Electrician, April 1894. 

 Phil Mag. S. 5. Vol. 40. No. 245. Oct. 1895. 2 B 



