1891.] Prof. Ewing on the Molecular Process in Magnetic Induction. 387 



WEEKLY EVENING MEETING, 

 Friday, May 22, 1891. 



David Edward Hughes, Esq. F.E.S. Vice-President, in the Chair. 



Professor J, A. Ewing, M.A. B.Sc. F.E.S. 



PROFESSOR OF APPLIED MECHANICS AND MECHANISM IN THE UNIVERSITY OF CAMBRIDGE. 



The Molecular Process in Magnetic Induction. 



Magnetic induction is the name given by Faraday to the act of 

 becoming magnetised, which certain substances perform when they 

 are placed in a magnetic field. A magnetic field is the region near 

 a magnet, or near a conductor conveying an electric current. 

 Throughout such a region there is what is called magnetic force, and 

 when certain substances are placed in the magnetic field the magnetic 

 force causes them to become magnetised by magnetic induction. An 

 effective way of producing a magnetic field is to wind a conducting 

 wire into a coil, and pass a current through the wire. Within the 

 coil we have a region of comparatively strong magnetic force, and 

 when a piece of iron is placed there it may be strongly magnetised. 

 Not all substances possess this property. Put a piece of wood or 

 stone or copper or silver into the field, and nothing noteworthy 

 happens ; but put a piece of iron or nickel or cobalt and at once you 

 find that the piece has become a magnet. These three metals, with 

 some of their alloys and compounds, stand out from all other sub- 

 stances in this respect. Not only are they capable of magnetic 

 induction — of becoming magnets while exposed to the action of the 

 magnetic field — but when withdrawn from the field they are found to 

 retain a part of the magnetism they acquired. They all show this 

 property of retentiveness, more or less. In some of them this 

 residual magnetism is feebly held, and may be shaken out or other- 

 wise removed without difficulty. In others, notably in some steels, 

 it is very persistent, and the fact is taken advantage of in the 

 manufacture of permanent magnets, which are simply bars of steel, 

 of proper quality, which have been subjected to the action of a strong 

 magnetic field. Of all substances, soft iron is the most susceptible to 

 the action of the field. It can also, under favourable conditions, 

 retain, when taken out of the field, a very large fraction of the 

 magnetism that has been induced — more than nine-tenths — more, 

 indeed, than is retained by steel ; but its hold of this residual mag- 

 netism is not firm, and for that reason it will not serve as a material 

 for permanent magnets. My purpose to-night is to give some account 

 of the molecular process through which we may conceive magnetic 

 induction to take place, and of the structure which makes residual 

 magnetism possible. 



