MAGNETISM AND TWIST IN IRON AND NICKEL. 



531 



wire, we can say nothing as to the probable result of any given combination of magnetic 

 force and mechanical stress. On the other hand, experiments on the magnetic effects 

 of twist may lead to some clearer knowledge regarding the magnetic distribution in the 

 heart of a wire. 



28. Take, for example, the following experiments, which well illustrate the extra- 

 ordinary complexity in the magnetic distribution of a nickel wire that is being twisted 

 or has been twisted. The wire, after being heated to a dull red heat, was placed 

 horizontally along the core of a magnetising coil, which extended five or six centimetres 

 beyond each end of the wire. The coil lay north and south, so that the wire was 

 magnetised inductively by the earth's horizontal force. A magnetometer was placed as 

 near as possible to the coil, and opposite the position that was to be occupied by the one 

 end of the nickel wire. This end was brazed to a stout brass rod which was clamped to 

 the magnetising coil. The other end was similarly fastened to a copper wire, which was 

 long enough to be gripped by a twisting apparatus set up close to the end of the coil. 

 The direct electromagnetic action of the magnetising current upon the magnetometer 

 was balanced by the action of a flat coil, included in the same circuit and suitably 

 adjusted. 



The deflections of the magnetometer magnet were measured by means of a beam of 

 light reflected in the usual manner from a mirror. The scale over which the spot of 

 light moved was divided to half millimetres, and was situated at a distance of 110 

 centimetres from the magnetometer. 



The purpose of the experiment was to study the effect of twist upon a nickel wire, 

 which, so far as the magnetometer indications could tell, was demagnetised. 



What follows is simply an account of the experiment as conducted, and the 

 deflections are the unreduced scale readings. 



Magnetometer zero 

 Nickel wire put into coil 

 Adjustment of resistance in coil circuit 

 just demagnetises the wire 



[I call this first current 100, and 

 - 45° twist applied 





 + 45 

 

 -45 

 

 Current off 

 Wire tapped 

 After another twist cycle and tapping 

 Current + 30, wire tapped 

 Current +44, „ „ 

 Current - 44, „ „ 

 Current + 30, „ „ 



until the negative current made to flow 

 express all subsequent currents in terms 



Deflection. 



589 



414 



589 



of it.] 



813 



824 



about 1200* 



935 



947 



about 1090 



about 1020 



816 



758 



700 



400 



620 



599 



* Readings above 1000 are rough estimates ; the spot of light was off the scale. 



