512 



Scientific Proceedings, Royal Dublin Society. 



Tablk V. 



From the curves in fig. 5 it will be seen that, unlike the iron wire, the 

 maximum twists with the nickel take place in the same longitudinal magnetic 

 field of about 13 c.g.s. units. 



If for any field we plot the values of the current density in the wire as 

 abseissse, and as ordinates the corresponding values of the twist, the points 

 will be found to lie, in every case, very approximately in a straight line, which, 

 when produced, passes through the origin, showing that the twist of the free 

 end of the wire is directly proportional to the current densit}^ in the wire. 



Experiments were now made in order to find out how the twist of the free 

 end of the wire varied when the cross-sectional area of the wire was changed. 

 Five wires were taken — Nos. 15 to 20, inclusive — and prepared to have the 

 degree of hardness Ho. Each wire was raised to a bright cherry-red heat 



