1895.] Tlic Effects of Electric Currents in Iron. 539 



WEEKLY EVENING MEETING, 

 Friday, April 26, 1895. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary 

 Secretary and Vice-President, in the Chair. 



John Hopkinson, Esq. M.A. D.Sc. F.R.S. M.B.L 



The Effects of Electric Currents in Iron on its Magnetisation. 



Let us recall a well-known experiment of Faraday's. Upon a ring of 

 iron is wound a few turns of copper wire, through which may be 

 passed a current. On it is a so wound a second coil, the ends of 

 which are connected to a galvanometer. It was known that when a 

 current is passed through the first-named or primary coil, the ring 

 becomes a magnet, and that if the current is reversed in direction the 

 magnetisation is also reversed. Faraday showed that when the 

 magnetism is reversed, a transient current is caused in the second 

 coil. The current which is reversed is a measure of the magnetising 

 force ; the total amount of current caused in the second coil by the 

 reversal is a measure of the induction of the magnet. The current 

 in the second coil is opposed in direction to the current in the 

 primary after reversal — that is, it is in the direction of the current 

 before reversal. If the changes in magnetisation in the iron produce 

 a current in the coil which is connected to the galvanometer, it is 

 clear that they will also tend to produce a current in the coil which 

 in like manner surrounds the magnet, and which has been connected 

 to the battery. This current is what used to be known as the extra 

 current. It continues for a short time the current in the battery coil 

 in the direction which it had before reversal, and retards its change 

 to the opposite direction. If the battery which gives the current is a 

 battery of many cells, and its extra E.M.F. is taken up by ordinary 

 non-inductive resistances, there will be an E.M.F. in the battery to 

 overcome these currents produced by the change of magnetisation, 

 and the reversal of the current will be effected speedily. If, however, 

 the number of cells is only sufficient to produce a current through the 

 resistances of the electromagnet, the change will be much slower. 



I have here an ordinary Westinghouse transformer which has an 

 electromagnet in the ring form, but the iron of its core is divided into 

 thin plates for the purpose of preventing currents in the iron itself. 

 There is upon it a primary coil connected to the battery and a 

 secondary coil connected to the galvanometer. I will first show you 

 the effect produced when a battery current is produced by a battery 



