GENERAL ACCOUNT OF MAGNETIC ACTIONS 191 



The second case which we shall consider is one discovered by 

 G. Wiedemann. If an iron wire is carrying an electric current, then 

 the lines of force are circles round the axis of the wire and the 

 wire is circularly magnetised, but as the lines of force are complete 

 within the wire, the iron does not produce an external field. But 

 Wiedemann found that if the wire is twisted it becomes longitu- 

 dinally magnetised. If the current is flowing down the wire and 

 the lower end is twisted so that the front part moves from right to 

 left, that lower end becomes a NSP. Suppose that the square 

 A B C D represents the unsheared form of a piece of the metal in 

 the front part of the wire, A D being parallel to the axis of the 



wire and the direction of the current. The magnetising force H due 

 to the current will be perpendicular as to A D. Now let the wire be 

 twisted so that C D moves to the left relatively to A B. This 

 shear may be represented by a lengthening of the diagonal A C 

 and of all lines parallel to it, and a shortening of the diagonal 

 B D and all lines parallel to it. With weak magnetisation the 

 permeability is increased along AC, and is decreased along B D. 

 The magnetising force due to the current is still H. Resolve it 

 into Hj parallel to A C, and H 2 parallel to D B. These are each 

 inclined at 45 t'o H, and are equal. But H r acting in a direction 

 of greater permeability, produces stronger magnetisation along A C 

 than H 2 produces along D B. The resultant magnetisation, there- 

 fore, has a component downwards. For if M 1 M 2 represent these 

 magnetisations, their resultant M slants down. Hence the lower 

 end of the wire has a NSP. 



Again we have a reciprocal effect. If the wire be magnetised 

 longitudinally and a current be passed along it, then the lower end 

 twists round. 



We might expect that with strong magnetising current these 

 effects would be reversed owing to the Villari reversal, but so far 

 they have always been found to be in the direction described, 

 perhaps because the intensity of magnetisation has not reached 

 t he critical point. 



