Bkown — Note on the Change of Length in Nickel Wire. 299 



the solenoid, and the hair in the microscope eye-piece set on the given mark ; 

 the magnetic field was then taken off, and the hair again moved to the mark ; 

 the diflference then gave the contraction of the wire for that magnetic field ; 

 that is, the contracted length of the wire was taken as the normal length in 

 «ach ease, and the apparent expansion of the wire as seen by the microscope 

 was measured, when the magnetic field was off. 



In order to compare the results obtained with alternating magnetic fields, 

 measurements were made with direct magnetic fields, and the values obtained 

 Avith the latter fields agree very well with those got by Honda and Terada, 

 and show that though the loads or tensions on the wire used in the present 

 experiments were smaller than tlie loads used by these Experimenters, the 

 softness of the wires used in the two cases was nearly the same. 



The alternating magnetic fields used, and the results obtained with them 

 recorded in this paper, were all at a frequency of 50 per second, and are here 

 expressed as root-mean-square values. 



If the results were to be expressed in terms of the maximum values of the 

 alternating magnetic fields, the effect on the A. 0. curve in the figure would 

 be to swing it to the right a little, and to raise it slightly. 



Alternating magnetic fields of frequency 100 per second were also tried 

 on the wire ; the results, however, are not recorded here ; but when they were 

 plotted "in a curve, it lay slightly below the curve obtained with the same 

 load on the wire, and with fields of frequency 50 per second. 



The results obtained with the three different loads, for both direct and 



