Brown — Mechanical Stress and Magnetisation of Iron. Ill 



in column marked oa are angles of twist where 1 cm. represents 20°, as in 

 fig. 3 ; but in the curves, fig. 5, four cms. are taken to represent 20° of twist, 

 so as to avoid a second set of numbers on the axis of ordinates. That is, the 

 numbers ou the axis of ordinates represent deflections on the galvanometer 

 scale for the curves nm and ob, and represent angles of twist in degrees for 

 the curve oa. 



The mil and ob curves are practically parallel up to a load of about 

 3 X 10^ grammes per sq. cm.; and as the load increases from about 10^ grammes 

 per sq. cm., these quantities decrease at a uniform rate — i.e. they diminish by 

 about 36 per cent, when the load is increased three times. In the curve marked 



SO 



4-X 70^ 



stress Grammes per sq. cm. 

 Fig. 5. 



oa, for the same increase in the load, the rate of decrease of tlie quantity 

 represented is smaller, and the diminution about 20 per cent. 



In order to test the effect of increasing the cross-sectional area of the 

 wire, and keeping the longitudinal magnetisation and the stress per unit 

 area constant, six new sets of wires were obtained of standard gauge, 

 Nos. 12, 14, 16, 18, 20, and 22. These wires were of the best Swedish 

 charcoal iron, and were delivered in perfectly straight lengths of about 

 9 feet, and all in the same physical state with respect to heat treatment. 



An extensive series of cycles were taken with these wires, as in the 



u2 



