Brown — Mechanical Stress and Magnetisation of Iron. 493 



If from the above table we plot as abscissae the values of the longitudinal 

 magnetic fields round the wire, and as ordiuates the corresponding values of 

 the twist in each of tlie three cases, we obtain three smooth curves which rise 

 gradually to a maximum, and then fall away more slowly. The maximum 

 twist occurs in different magnetic fields in the three cases — viz., about 2'5, 5'5, 

 and 11 units respectively ; and if we further plot the values of the current 

 through the wire as abscissse, and as ordiaates the corresponding values of 

 the magnetic field in whicli the maximum twist occurs, the three pohits will 

 be found to lie very approximately in a straight line, which line, when 

 produced, passes through tlie origin. This shows that the longitudinal 

 magnetic field in which the maximum twist occurs is directly proportional to 

 the current density in the wire. 



The results — collected from the tables above — for wires of very 

 approximately the same diameter, but of different degrees of magnetic 

 softness, are shown as curves in fig. 4. 



]<'iG. 4. — Magnetic field. 



All these wires were of the full length, 226 cm., and all loaded at the 

 rate of 0'5 x 10^ grammes per sq, cm. The top curve is that obtained with 

 the softest wire H^, and the lowest curve that obtained with the hardest wire 

 H3 ; the intermediate curves are those obtained with the wires of intermediate 

 hardness Hi, Hj, and H2.5 respectively. 



The wires at the different tempers were very approximately of the same 

 diameter (Table I., p. 482), and the maximum twist occurred in all cases 

 when the wire was in a longitudinal magnetic field of 2'5 c.g.s. units. 



