Brown — -The Fatigue of Nickel and Iron Wires. SS?" 



because it has also been sliown^ that the greatest damping of torsional 

 oscillations in nickel wire takes place under these same conditions, and it 

 follows that the maximum fatigue will also occur in the same circumstances. 



Tlie solenoid used in these experiments has already been described.' The 

 only important change in the apparatus is an improvement in the method of 

 applying the load on the lower end of the wire. The lead discs formerly 

 employed have been replaced by lead cylinders which can be clamped firmly 

 by means of lock-nuts, so that no change of zero can take place by the vibra- 

 tions wliich are set up when the alternating currents are round the solenoid. 



The fatigue does not occur, or only very slightly, in soft nickel wire. In 

 order therefore to get some idea of the limits between which it does take 

 place, the simple rigidity was measured in the three cases liere given. Tiiis 

 was done by means of the modified form of Searl's Torsion apparatus 

 previously described.' 



The nickel wires employed were eacli 226 cms. long and 0'1675 ems. in 

 diameter, and the millimetre scale for reading off the steady deflection or 

 twist of the free end of the wire was placed at a distance of 167 cms. from 

 the plane mirror on the vibrator or load on the end of the wire. The method 

 of experiment was as follows : — Tlie nickel wire was suspended vertically 

 in the middle of the solenoid ; it was firmly fixed to the wall at the top, and 

 the lower end made contact in a vessel of mercury by means of a small iron 

 pin projecting from tlie under side of the load. Then for a given 

 longitudinal load on the wire, the corresponding direct current was [lut 

 through the solenoid to give the magnetic field which produces the largest 

 twist of the lower end of the wire when tlie standard current of one ampere 

 was sent through it ; this gave a steady deflection of the light spot on the 

 scale which we call D. The direct currents were then taken oil both the 

 wire and tlie solenoid, and the equivalent root-mean-square value of alter- 

 nating current was for 0)ie minute sent round the solenoid ; this alternating 

 current was then taken olf, and separate direct currents of the same values as 

 before were sent round the solenoid and through the wire, and the twist or 

 steady deflection of theiightspot on the scale again observed, which we call 

 d. The direct currents were again taken off the wire and solenoid, and the 

 alternating current again put round the solenoid for one minute more then 

 taken off, and the direct currents again put through the wire and round the 

 solenoid, and the deflection again observed, and so on until the twist or 

 deflection was no longer diminished by the application of tlie alternating 

 magnetic field. 



1 Scient. Proc. Roy. Dub. Soc, vol. iiii, p. 3d. 



'^ Ibid., vol. xiT, p. 216. ^ ]Hd., vol. xii, p. 481. 



3 H 2 



