530 



Prof. D. E. Hughes. 



[Mar. 17, 



current through the inducing coil no current is perceptible upon the 

 iron wire ; but if we give a very slight twist to this wire at its free 

 end — one-eighteenth of a turn, or 20° — we at once hear, clear and 

 comparatively loud, the currents passing the coil ; and although 

 we only gave a slight elastic twist of 20° of a whole turn, and this 

 spread over 20 centims. in length, making an extremely slight molar 

 spiral, yet the effects are more powerful than if, using a wire free from 

 stress, we turned the whole coil 40°. The current obtained when we 

 turn the coil, as just mentioned, is secondary, and with the coil at any 

 angle any current produced by its action, either on a copper, silver, 

 iron, or steel wire ; in fact, it is simply Faraday's discovery ; but the 

 current from an elastic twist is no longer secondary under the same 

 conditions, but tertiary, as I shall demonstrate later on. The current 

 passing through the coil cannot induce a current upon a wire perpen- 

 dicular to itself, but the molecules of the outside of the wire, being 

 under a greater elastic stress than the wire itself, they are no longer 

 perpendicular to the centre of the wire, and consequently they react 

 upon this wire as separate magnets would upon an adjacent wire. It 

 might here be readily supposed that a wire having several twists, or a 

 fixed molar twist of a given amount, would produce similar effects. 

 It, however, does not, for in most cases the current obtained from the 

 molar twists are in a contrary direction to that of the elastic torsion. 

 Thus, if I place an iron wire under a right-handed elastic twist of 20°, 

 I find a positive current of 50° sonometer ; but if I continue this twist 

 so that the index makes one or several entire revolutions, thus giving 

 a permanent molar twist of several turns, I find upon leaving the 

 index free from any elastic torsion, that I have a permanent current of 

 10°, but it is no longer positive, but negative, requiring that we should 

 give an elastic torsion in the previous direction, in order to produce a 

 positive current. Here a permanent elastic torsion of the molecules 

 is set up in the contrary direction to its molar twist, and we have a 

 negative current, overpowering any positive current which should 

 have been due to the twisted wire. 



The following table shows the influence of a permanent twist, and 

 that the current obtained when the wire was freed from its elastic tor- 

 sion was in opposition to that which should have been produced by the 

 permanent twist. Thus, a well-softened iron wire, 1 millim. in dia- 

 meter, giving 60° positive current for a right-handed elastic torsion of 

 20°, gave after l o, 80 permanent torsion a negative current of 10°. 



1 complete permanent torsion (right-handed) negative .... 10 



2 „ ,, „ .... 15 



3 „ ,, „ .... 15 



4 ,, ,, .... 16 



5 „ „ 7 , , ... 1 2 



