Brown — Mechanical Stress and Magnetisation of Iron. 113 



ones ; on the axis of abscissae 1 cm. still represents 20° of twist, but on the 

 axis of ordinates 1 cm. represents 10 divisions on the galvanometer scale 

 instead of 4 cms., as in the previous cases. 



In curving both sets of observations a scale was chosen, so as to get a 

 minimum of error in counting the squares for finding the areas of the cyclic 

 curves. 



As already stated, each wire before being tested was annealed with a 

 bunsen flame, then carefully cleaned with emery-cloth, and hung up in the 

 Earth's vertical field of 045 c.g.s. units as indicated in fig. 1. 



The throws on the galvanometer scale were observed, as before, for the 

 first part of the cycle by going steps of 20° each from 0° to 180° ; the 

 galvanometer was then cut out of circuit, and the wire put through four 

 complete cycles ; the galvanometer was put in circuit again, and the cycle 

 completed in the usual way. 



All the six sets of wires were done in this manner with a longitudinal 

 stress or load on of 10^ grammes per sq. cm., and in a magnetic field of 

 0'45 c.g.s. unit. 



The results are here given in Table V., and shown in the curve fig. 7. 



Table V. 



Since the area of the cyclic curves represents in arbitrary units the 

 amount of circular magnetism produced by the twist, we find that for wires 

 up to about 15 X 10"^ sq. cms. cross-sectional area, the circular magnetism 

 is very nearly proportional to the sectional area of the wire, and from 

 24 X 10"' to 54 X 10"' sq. ems. the change is very small. The latter is due 

 to the fact that these two wires were permanently deformed when the angle 

 of twist was 180°, i.e. at the top part of the cycle. This was observed, 

 as when the cycles for these two wires were finished, and the vibrator left 



