42 



Scientific Proceedings, Royal Dublin Society. 



means of the microscope ; and in order to avoid as much as possible any error 

 due to this heating of the wire, the following method of taking the readings 

 was adopted : — The magnetic field was put on the solenoid — and kept on as 

 short a time as possible — and the hair in the eye-piece of the microscope set 

 after a few trials on the given mark on the wire, then, before the second or 

 zero reading was taken, an interval of from 2 to 12 minutes was allowed to 

 elapse, so that the expansion by heat due to the magnetic field was avoided. 



This heating of the wire by an alternating magnetic field of frequency 

 250 per second was so great that experiments with this magnetic field were 

 not continued. Moreover, it was considered that the results obtained with 

 an alternating magnetic field of frequency 150 per second indicated what 

 might be expected from applying the higher frequency magnetic field. 



In order to compare the results obtained in the case of alternating 

 magnetic fields, with those obtained with direct magnetic fields, observations 

 were made up to a maximum value of 200 c.g.s. units in both cases ; the 

 alternating magnetic fields are expressed as root-mean-square values. 



The results obtained with the wire having a simple rigidity of about 

 810 x 10 6 grammes per sq. cm. are shown in Tables I, II, and III, when here, 

 as well as throughout the paper, H stands for the strength of the applied 

 magnetic field, whether longitudinal (D.C.) or alternating (A.C.), and n for 

 the frequency of the alternating field. 



Table I. 

 Eigidity =? 810 x 10 6 grammes per sq. em. 

 Load = 0T25xl0 5 „ „ „ „ 



