300 



Scientific Proceedings, Royal Dublin Socieiy. 



alternating magnetic fields, are shown in the table, when the D. C. value& 

 and the A. C. values are placed side by side under the respective loads on 

 the wire, each load beiug expressed as grammes weight per square 

 centimetre. 



The middle values, or those obtained with the load 0"5 x 10° grammes 

 per sq. cm., are plotted as curves in the figure where the abscissae are the 

 values of H, the magnetic field round the wire, and the ordinates the corre- 

 sponding values of the contraction of the wire expressed in millionths per 

 unit lengtli. 



■^K 



Comparing the last values in the columns of the table — that is, at the 

 magnetic fields 200 c.g.s. units, or just about where the curves, when plotted, 

 tend to become horizontal, we find that the amount of contraction in nickel 

 wire for tlie alternating magnetic fields is greater than for the direct 

 magnetic fields by about 63 per cent, for the smallest load and about 44 per 

 cent, for the largest load here employed. Neglecting the values in the table 

 for the low magnetic fields, it will be seen that tlie contraction for a given 

 root-mean-square alternating field is nearly in a constant ratio to the 



