102 Scientific Proceedings, Royal Dublin Society. 



These general statements are necessarily qualitative, and when taken in 

 pairs as marked they are complementary to each other ; for instance, in the 

 first or 1, by the application of magnetic forces a mechanical result is 

 obtained, and in Iff, a mechanical twist is applied with magnetic effects. The 

 same holds with the other two pairs of statements. 



In what follows an attempt has been made to measure quantitatively the 

 effects produced by varying the quantities above mentioned. 



Now it is well known that the transitory currents set up in a longi- 

 tudinally magnetised iron wire are due to the circular magnetisations 

 produced by twisting the wire ; and Professor Ewiug' in 1883 showed how 

 circular magnetisation exhibits hysteresis with regard to the angle of twist. 

 It occurred to me some time ago that this might be a means of finding, 

 among other things, a relation between the longitudinal stress per unit area 

 on the wire and the circular magnetisation produced by the twist. 



As far as I know, this has not been directly touched upon in any of the 

 papers on this subject, to the number of seventy or so, that have been pub- 

 lished before or since 1900. 



I have therefore in this paper brought together some results obtained 

 witli iron wires, showing the effects of keeping : — 



1. The longitudinal magnetisation and the cross-sectional area of the 



wire constant, and varying the longitudinal stress per unit area. 



2. Tlie longitudinal magnetisation and the stress per unit area constant, 



and varying the cross-sectional area of the wire. 



3. The longitudinal magnetisation and cross-sectional area of the wire 



constant, and varying the circular magnetisation and the longi- 

 tudinal stress per unit area. 



4. The longitudinal magnetisation and the stress per unit area constant, 



and varying the cross-sectional area of the wire and the circular 

 magnetisation. 

 The arrangement of the apparatus used is shown in fig. 1. 



BV \s, an iron wire suspended in the vertical component of tlie Earth's 

 magnetic field. The top end B is fixed to an insulated support on a cross- 

 beam in the ceiling of the room, and the lower end, about 9 feet below, 

 dips into a mercury cup C. 



On the lower end of the wire is fixed a non-magnetic cylinder or weight 

 made of brass and lead, which has attached to it two light handles or pointers 

 P, by means of which the required twist could be given to the vertical wire. 



1 Proc. Eoj'. Soc, London, 18S3, also " Magnetic Induction in Iron and other Metals," 3rd ed., 

 p. 237. 



