MAGNETISM AND TWIST IN IRON AND NICKEL. 497 



140 or 150 units the magnetic contraction is increased by tensional stress up to a certain 

 critical value. This critical value lies somewhere between 600 and 1000 kilos, per square 

 centimetre, a result which is in good agreement with my own results as indicated in 

 curves 2, 4, 5. There is also a distinct resemblance in the manner in which tension 

 affects the form of the curves of contraction and twist obtained respectively by Mr 

 Bidwell and myself. 



It seems to me that here we have a further corroboration of the truth of Maxwell's 

 explanation of the Wiedemann effect. As I pointed out in my first paper, Professor 

 Barrett's discovery of the contraction of nickel in magnetic fields at once suggested that 

 the Wiedemann effect in nickel should be opposite to that in iron. Experiment fully 

 bore this out. Then, again, the peculiarity of the influence of tension in the Wiedemann 

 effect in nickel suggested that a similar peculiarity would be found to exist in the 

 influence of tension on the contraction effect ; and this Mr Bidwell has completely 

 verified. In short, the more we study the details of the Joule and Wiedemann 

 magneto-strain effects in iron and nickel, the more are we convinced of their intimate 

 connection. 



Another feature common to the iron and nickel is the wider distance apart of the 

 current-reversal and field-reversal curves, when the line current is taken smaller. This 

 feature is particularly noticeable in the case of nickel, for which, in some instances in 

 which the line current is strong (e.g., Nos. 3 and 4), the curves nearly coincide in the 

 higher fields. 



8. Numerical Comparison of the Twists in Iron and Nickel. — In Part I. # I obtained 

 an expression for the twist in an iron or nickel tube under the combined influence of 

 circular and longitudinal magnetising forces. This expression was 



2(^-0-0 a(3 

 u ~ r « 2 +/3 2 



in which 6 is the twist per unit length of the tube, r is the radius, a and f3 are the two 

 magnetising forces, rs is the elongation in the direction of the resultant magnetising 

 force, and or the elongation in directions at right angles thereto. This expression was 

 obtained on the assumption that the direction of maximum elongation in a strained 

 element coincides with the direction of the resultant magnetising force. We shall discuss 

 the merits of this assumption immediately. Meanwhile let us compare the quantities 

 (tSi - (Ti) for iron and nickel wires ; or, more strictly, let us compare the unknown 

 multipliers (k) in the expression 



e= k a P 



r a 2 + /3 2 



Take the iron curve No. 3, Plate I., and the nickel curve No. 1, Plate II. The greatest 



* It may be well to point out that in equation (5), on page 388, the letter 6 is inadvertently used in two 

 different senses. Since, however, the one 6 is brought in for a single transformation, there is no confusion in 

 the final result. 



