MAGNETISM AND TWIST IN IKON AND NICKEL. 



491 



pair of curves. This " point of equal twists," as I shall call it for shortness' sake, occurs 

 in higher fields for higher values of the line current, other things being the same. 

 Compare, for example, Nos. 1, 2, 3, 4, 11, 12, or Nos. 7, 8, 9, or Nos. 6 and 10. On the 

 other hand, the effect of increasing tension is to bring the point of equal twists into 

 lower fields. This may be inferred from a comparison of Nos. 4, 5, 8, 10, or Nos. 2, 6, 7, 

 in which groups the line currents are near enough in value to enable us to make the 

 comparison. The following table will bring out the remarkable simplicity of the relations 

 connecting the value of the field for equal twists with the tension and the line current. 

 The last column gives the ratio of the field to the current. 



Tension. 



Current. 



Field for 

 Equal Twists. 



Eatio. 



907 



1-06 



4 



3-77 



907 



1-2 



4-2 



3-50 



907 



2 - 2 



5-9 



2-71 



907 



3-04 



9-5 



3-11 



907 



314 



9 



2-85 



907 



3-84 



11-5 



292 



1814 



3-78 



7-5 



1-98 



2721 



2-8 



4-5 



1-60 



2721 



332 



5-2 



1-56 



3537 



1-74 



1 





3537 



2-6 



35 



1-34 



3537 



352 



4-5 



1-28 



For each group with the same tension, there is an approximately linear relation 

 connecting current and field for the point of equal twists ; and as the tension increases 

 the ratio of current to field distinctly falls off. This, however, we may regard as a 

 secondary aspect of the general effect of increasing tension. As already mentioned, the 

 increase of tension causes a decrease of twist ; and, by a study both of the table and the 

 curves, we may readily establish that under increased tension the point at which the 

 curve cuts through the axis of co-ordinates is shifted backwards towards the origin. This 

 is the point of vanishing twist, which, again, shifts into higher fields when the line 

 current is taken stronger. Thus in every case the effect of a stronger current is opposite 

 to the effect of a greater tension. The shifting of the point of vanishing twist into lower 

 fields when the tension is increased is, if we are to explain the Wiedemann effect in 

 terms of the Joule effect, in good accord with Mr Bidwell's results. He found that 

 under greater tension iron begins to contract in lower magnetic fields — i.e., the field for 

 no change of length was smaller at the higher tensions. 



We know nothing so far regarding the changes of length when an iron wire carrying 

 a current is subjected to longitudinal magnetising forces. But from the fact just 

 established that an increased current along the wire affects the point of vanishing twist 

 in a manner opposite to that in which an increased tension affects it, we should be 

 inclined to conclude that the pure strain effects of these influences are of an opposite 

 character. The strain produced by a current along a wire has not been directly studied ; 



