Bkown — The Fatigue of Nickel and Iron Wires. 



341 



— that is, between being too hard and too soft — are so narrow, that it is difBcult 

 to judge how much the wire should be heated in order to obtain a desired 

 rigidity. In the three attempts liere recorded the heating has just happened 

 to give very nearly the rigidities required, and to show the approximate 

 relations between the longitudinal load on the wire, tlie rigidity, and the 

 corresponding maximum fatigue; the values are here collected in Table IV. 



Table IV. 



Section 2 



Iron Wires. 



In the case of an iron wire of given diameter it is known — (1)^ that the 

 continuous direct magnetic field which must be round the wire in order to 

 obtain the maximum twist or " Wiedemann effect " is independent of the 

 longitudinal load on the free end of the wire for a given current through 

 it, (2)* that tlie direct continuous magnetic fields in which the greatest internal 

 friction — or opposition to torsional oscillations of the wire — takes place, are 

 much lower for iron than for nickel wires, and the points of maximum effect 

 are also mucli less pronounced.^ From these considerations one would therefore 

 expect that iron vfire would not be so easily fatigued by the application of 

 alternating magnetic fields as nickel wire ; in fact, the results of experiments 

 here recorded show that the expectation is justified. 



' Scient. Proc. Roy. Dub. Soc, vol. xii, p. 484. ^ Ibid., vol. siii, p. 41. 



' The influence of alternating magnetic fields on the torsional oscillations of iron wire is at 

 present under investigation. 



