298 Seientific Proceedings^ Royal Dublin Society. 



length of the soft nickel wire employed was 226 cms., its diameter being 

 1'675 mm., and it was in a uniform magnetic field throughout the entire 

 length. 



The magnetic fields applied, both direct and alternating, were up to 

 200 c.g.s. units as a maximum, and the three loads or tensions employed 

 were (01184, 0'5, and 1) x 10^ grammes per sq. cm. respectively. The 

 loads were applied by means of a three-jaw clutch which gripped the lower 

 end of the wire, and then a cord passing over two frictionless pulleys had 

 a scale-pan on its end, in which were placed the weights. This method 

 gave a perfectly steady tension on the wire, and at the same time kept it 

 centrally inside the solenoid. The top end of the wire was caught by a 

 similar three-jaw clutch, and firmly secured to the wall. On account of the 

 great length of the wire under test, the change of length or contraction on 

 application of the magnetic fields could be read off directly, by means of a 

 reading microscope having a fine hair in the eye-piece. The movement of the 

 hair was made by means of a micrometer screw witii graduated head or cap ; 

 the circumference of the cap was divided into 100 divisions, and one whole 

 turn gave a motion to the hair of 0-0208 cms. ; therefore one division on the 

 cap corresponded to a motion of 0'000208 cms. ; or 9-2 x lO"' per unit length 

 of the wire. By turning the micrometer always in the one direction during 

 the measurements, so as to avoid any small back-lash on the thread, it was 

 found that the same readings for a given magnetic field could be obtained 

 repeatedly. 



The temperature of the room during the experiment was kept as uniform 

 as possible, at about 17° 0. : it was found, however, that when the longitu- 

 dinal continuous magnetic field was round the wire, the wire became slightly 

 warmed, as shown by the slight elongation observed in the microscope ; and 

 this heating of the wire was more pronounced when an alternating magnetic 

 field was applied. For example, when a direct magnetic field of 166 c.g.s. 

 units was put on for 5 seconds, and then taken off, the wire elongated for 

 about 5 minutes to the extent of 7 divisions on the cap of the micrometer 



screw, or of value — = 6'44 x 10"" cms. ; and when an alternating magnetic 



field of root-mean-square value 156 c.g.s. units was put on for 10 seconds 

 and then taken off, the elongation of the wire went on for about 5 minutes 



and then stopped, the amount of the expansion being — = 14*7 x 10"° cms. 



In order to avoid as much as possible any error in the readings of the 

 microscope due to this heating of the wire by the magnetic fields, the following 

 method of taking the readings was adopted : — The magnetic field was put on 



