110 



Scientific Proceedings, Royal Dublin Society. 



cm., the tenacity or breaking weight of iron. Beyond this point the rate of 

 diminution is slower ; and if the curve were continued, it would meet the 

 axis in the figure approximately at the point 7'5 x 10^ It was found by 

 a separate experiment that the elastic limit of this wire was reached with a 

 load of 16*5 kilos, or about 8 x 10^ grammes per sq. cm. When the curve is 

 plotted on a larger scale, with both the axes starting from zero, a tangent 

 drawn from that point on the axis of abseissse representing the tenacity of 

 iron (i.e. 65 x 10*) just meets the curve at the point of flexure; whilst the 

 straight part of the curve produced meets the axis of ordiuates at the point 

 165. From this we might infer that maximum magnetisation would be 

 obtained with a minimum weight, and vice versa ; the straight part of the 

 curve, however, is the only part we are certain about, and this shows that, as 

 the stress per unit area increases from 10* to 2'5 x 10* grammes per sq. cm., or 

 250 per cent., the area of the cyclic curve or the circular magnetisation in 

 tlie iron wire decreases from 130 to 80, or about 40 per cent. 



Referring again to fig. 3 : the whole eight cyclic curves obtained were 

 plotted to the same scale as here used, and the distances on the curve marked 

 mn, ob, and oa were measured. The corresponding distances on an ordinary 

 H-B cyclic curve for iron are what determine its shape and area, so here we 

 get some interesting relations between these values on the circular magnetisa- 

 tion curves when they are plotted against the load on the wire. These 

 values are given in Table IV., and shown in curves in fig. 5. 



Table IV. 



The numbers in columns marked mn and ob are deflections on the 

 galvanometer scale where 2 cms. represent 10 divisions, and the numbers 



