Brown — Mechanical Stress and Magnetisation of Nickel. 29 



the wire, the maximum value of which was 100 amperes per square centimetre ; 

 and the twist of the lower free end of the wire was read off by means of the 

 usual mirror, lamp, and scale arrangement. The scale was divided into 

 millimetres ; and its distance from the mirror on the vibrator^ at the lower 

 end of the wire was 116'5 cms. 



With a certain longitudinal load on the wire, the twists of its free end were 

 observed when tioenty-six different values of longitudinal magnetic fields were 

 successively applied ; the load on the wire was then increased, and the twists 

 were again observed for the same magnetic fields ; and so on for six loads in 

 all, the highest load being about 13'3 kilos, or at the rate of 6 x 10' grammes 

 per square centimetre. 



The results obtained with the six different longitudinal loads, and when 

 the wire was placed in longitudinal magnetic fields up to a maximum of 200 

 c.g.s units, are given in Table I., and are shown as curves up to a magnetic 

 field of 100 units in fig. 1, p. 31. 



From the curves it will be seen that the maximum twist attains its highest 

 value when tlie longitudinal load on the wire is at the rate of 4 x 10* grammes 

 per square centimetre, and for the two higher loads the maximum twist is 

 about the same, or slightly diminished. The arrangement of the apparatus 

 would not allow with safety the application of still higher loads. 



If we plot on the axis of abscissae the values of the longitudinal load on 

 the wire, and as ordinates the corresponding values of the longitudinal 

 magnetic field in which the maximum twist occurs, the points will be found to 

 lie very approximately in a straight line, showing that, between these limits of 

 the load, the magnetic field in which the maximum twist takes place is 

 proportional to the load at the end of tlie wire. 



From Table I., by plotting as abscissae the value of the longitudinal load 

 on the wire, and as ordinates the corresponding values of the twist for any 

 longitudinal magnetic field between 70 and 200 c.g.s. units, tlie points in 

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

 that when the twists have attained their highest values, between the limits of 

 these loads, the curves are practically parallel for high magnetic fields. 



It will be noticed in fig. 1 that, as the load on the wire is increased, the 

 peak or turning-point of the curve becomes less sharply defined ; and in order 

 to bring out more clearly the effect of an increased load on the magnetisation 

 of the wire, a series of cyclic curves were taken when the wire was surrounded 

 by the magnetic field corresponding to the maximum twist for a given load 



' The vibrator which constituted the load ou the wire in all cases was composed of brass and 

 lead, so as to avoid errors due to its becoming magnetised, 



£ 2 



