Brown — Meclmnical Stress and Magnetisation oj Nicltel. 609 



Again, by plotting the values of the load on tlie wire as abscissae, and as 

 ordiuates the corresponding values of the longitudinal magnetic fields 

 in whicli the maximum twist occurs, the points will be found to lie in a 

 straight line which, when produced, cuts the axis of ordinates at tlie point 

 10 on the scale. This indicates that, if the wire were tested with no 

 longitudinal load on it, the maximum twist would occur wlien a longitudinal 

 magnetic field of 10 units was round the wire. In order to test this 

 experimentally, a thin aluminium tube holding a mirror, and a piece of iron 

 wire for making contact with the mercury, was hung on the lower end of the 

 wire, the whole weighing only 33 grammes, giving a load per sq. cm. of 

 0-016 X lO'* grammes. The results of the test are shown in Table III, 

 in the first line under the length 181 cms. ; and wlien the curve is 

 drawn, the maximum twist occurs when the wire is surrounded by a 

 longitudinal magnetic field of 10-3 units, which practically confirms the 

 observation made above. 



Magnetic Field H. 

 Fig. 4.— Wire of constant temper and nnder three different loads. 



The curves relating magnetic field and twist in nickel wires, witli respect 

 to the longitudinal load, are different from those given by an iron wire, 

 inasmuch as, after a certain longitudinal magnetic field (about 24 units), the 

 twists produced on a highly loaded wire are greater than the twists given by 



4 c 2 



