Bkown — Mechanical Stress and Magnetisation of Iron. 181 



ordinates against tlie corresponding values of the cross -sectional area of the 

 wire as abscissae, these four points will also be found to lie practically on a 

 straight line ; and, as before, when the cross -sectional area of the wire is 

 increased about 4'7 times, the maximum circular maguetism is increased 

 2'5 times. 



In fig. 3 are plotted the longitudinal maguetic fields as abscissae, and as 

 ordinates the maximum transitory current obtained by twisting one end of 

 the wire through an angle of 100". As one would expect, these curves are of 

 very much the same form as those in fig. 2 : the value of tlie maximum 

 transitory current increases with the increase of the magnetic field round the 

 wire up to a certain value, and then diminishes as the field is furtlier 

 increased. 



Magnetic field ill c. g. s. units. 

 Fio. 3. 



As in fig. 2, the top curve iu fig. 3 is that obtained for the thickest wire, 

 and the lowest curve that for tlie wire of smallest cross-section : i.e., taking tlie 

 curves from the top downwards, they were obtained with wires Nos. 12, 14, 16, 

 18, and 20 respectively. 



If we pick off from tlie first four curves the values of the magnetic fields 

 which give the highest points on the curves, and plot these values as ordinates 

 against the corresponding values of the cross-sectional areas of tlie wires as 

 abscissae, the four points will be found to lie in a straight line. This shows 



