BiiOWN — Mechanical Stress and Magnetisation of Iron. 497 



Tablk IX. 



The results are shown in Table IX., and if these be plotted with the values 

 of the longitudinal magnetic fields as abscissae, and the corresponding values of 

 the twist as ordinates, the theoretical considerations are seen to be practically 

 confirmed, that is, that the twist is inversely proportional to the cross- 

 sectional area of the wire, or directly proportional to the current density in 

 the wire, for any given longitudinal magnetic field. This is seen when we 

 plot the values of the current density in the wire, against the corresponding 

 values of the twist ; the points will then be found to lie in a straight line which 

 when produced passes through the origin. Also, by plotting the values of 

 the cross-sectional area of the wire as abscissae, and the corresponding values 

 of the twist as ordinates, the points will lie in a curve which gradually 

 approaches the axis of abscissae. Since the twist varies inversely as the 

 radius of the wire squared, and the cross-sectional area of the wire varies as 

 the radius squared, the slope of the curve varies inversely as the radius of the 

 wire to the fourth power ; and therefore, as the radius increases, dijldx 

 decreases rapidly, and becomes zero at infinity. 



