49(5 



PROFESSOR KNOTT ON SOME RELATIONS BETWEEN 



Table III. — For Nickel Wire, 0'94 mm. in diameter — continued. 



Tension Kg.-Wt 

 per Sq. Cm. 



Line Current 

 in Amperes. 



Longitudinal 



Field Electrom. 



Units, C.G.S. 



Twist ± H in 



Thousandths of 



Radians. 



Twist ± C in 



Thousandths of 



Radians. 



Experiment 

 Number. 



T 



C 



H 



ht 



ct 



No. 







5-60 



178-8 



135-3 



88-0 



49-0 



29-7 



100 



7-0 



2-1 



21-2 



22-3 



23-5 



23-6 



20-4 



7-2 



4-1 



0-8 



17-7 

 18-5 

 19-4 

 18-8 

 17-2 

 11-7 

 8-5 

 2-9 



(13) 



It will be seen that the attainable twists in nickel are much greater than those in iron. 

 In high enough fields and with strong enough currents there is indeed no difficulty in 

 seeing the wire twist with the naked eye. 



7. Discussion of the Results for Nickel. — In many respects the features of the nickel 

 curves strongly resemble those of the iron curves. There are, for example, well-marked 

 maximum points ; and the fields in which these occur depend on the currents along the 

 wire, being higher for the stronger current. The current-reversal twist begins by being 

 greater than the field-reversal twist, but becomes smaller when the field is high enough. 

 The point of equal twists moves into higher fields as the line current is taken stronger ; 

 compare Nos. 1, 2, 3, and Nos. 5 and 6. On the other hand, the point of equal twists 

 moves into lower fields as the tension is increased. 



In some particulars, however, there are diiferences in the behaviour of the iron and 

 nickel. In the case of nickel, an increase of tension has a marked effect on the form of 

 the curve, and leaves a permanent change on the wire. Thus in No. 13 the twists are 

 smaller than in No. 2 — the only difference between the two experiments being that No. 

 2 was made before, and No. 13 after the wire had been subjected to a considerable 

 tension. Again, the effect of tension on the magnitude of the twist in higher fields is, 

 under certain circumstances, to increase it. Compare, for instance, Nos. 3 and 4 in Plate 

 II. Thus the first effect of an increasing tension is to increase the twist for a given 

 combination of magnetising forces — a conclusion already arrived at in my earlier paper 

 (see Part I.). Ultimately, however, as the tension is made greater and greater, the twist 

 begins to diminish ; compare curves 4 to 8. In my earlier paper (see p. 383), it was 

 not possible for me to compare this effect of tension on the twist in nickel with the effect 

 of tension on the contraction of nickel. Mr Bidwell has recently, however, given a 

 further instalment of his investigations on changes of length of magnetic metals, and has 

 found a peculiarity in the behaviour of nickel under tension very similar to what I have 

 more fully described in Part I. In a preliminary paper, published in the Proceedings of 

 the Royal Society (vol. xlvii. p. 467, July 17, 1890), he shows that in weak fields the 

 magnetic contraction of nickel is diminished by tension, but that in fields of more than 



