384 Mr. F. J. Smith on the Effects of Magnetism 



form of short notes. In the last two notes will be found 

 a description of the application of one of the phenomena to 

 the production of a chronograph stylus. I have added below, 

 a list of some of the papers which treat of relationships which 

 exist between stress and electromagnetic induction. 



1. In 1841 Joule showed that if an iron bar was magnetized 

 it was increased in length ; he also found that if an iron bar 

 was subjected to considerable tension and then magnetized it 

 was shortened. Since Joule's discovery the action of the force 

 of magnetization on iron subjected to stress has been investi- 

 gated by Matteucci, Compt. Rend. t. xxiv. p. 301 (1847) ; 

 Wertheim, ibid. xxxv. p. 702 (1852) ; also Ann de Chem. et 

 de Phys. [3], 1. 1. p. 385 (1857) ; Villari, Pogg. Ann. (1868) ; 

 Wiedemann, Galvanismus, p. 447 ; Sir W. Thomson, Phil. 

 Trans. 1879, p. 55 : Prof. Hughes, Proc. R. S. (1881), xxxi. 

 p. 524 ; id. ibid, xxxiii. p. 25 ; Mr. S. Bidwell, Proc. R. S. 

 xi. p. 257 (1886) ; ditto, xlvii. p. 496 ; Prof. Ewing, Proc. 

 Roy. Soc. vol. xxxvi. p. 117 ; ' Nature,' vols, xlii., xliii. 



2. Wiedemann, Galvanismus. p. 447, showed that under 

 certain conditions twisted iron wires untwist on magnetization. 

 His words are : — " Die gedrillten Eisendrahte detordiren sich 

 also bei der Magnetisirung." He made his experiments on an 

 iron wire suspended in a vertical position and weighted with 

 a considerable mass, viz. 1103 grammes. By means of a 

 disk attached to it, and a cord and weight, torsion was given 

 to the iron wire. A coil of copper wire, in the axis of which 

 was the iron wire, was used for magnetizing the latter ; change 

 of torsion was read by means of a mirror and scale seen in 

 the field of a telescope. In his experiments torsional set was not 

 produced, or if so in only a very small degree. In my own 

 work rods of iron, steel, and nickel which were not weighted 

 and had received permanent torsional set were experimented 

 on. The magnetizing force H used by me was many times 

 greater than that used by Wiedemann ; as far as I can calculate 

 probably in some cases thirty-five times as great. I arrived 

 at this conclusion by assuming his coil to have had next to no 

 resistance, also that the whole number of cells mentioned were 

 used by him. 



3. The first experiment was arranged to find out whether a 

 magnetic field of force acting on an iron rod only subject to its 

 own weight would set up a torsional stress in it. The follow- 

 ing apparatus was used : — A magnetizing coil 50 centim. long, 

 wound on a glass tube 0'825 centim. external diameter ; the 

 number of turns of silk-covered copper wire 0*6 millim. 

 diameter was 3865 ; the coil was wound so as to have no 

 component along its length, its magnetizing force for one 



