THE MOLECULAR PROCESS IN MAGNETIC INDUCTION.* 



By Prof. .1. A. EwiNG, F. R. S. 



Magnetic indiu'tioii is the name given by Faraday to the act of be- 

 comiug magnetized, wliieh certain substances perform when they are 

 ]»laced in a magnetic field. A magnetic field is the region near a mag- 

 net, or near a conductor c<mveying an electrics current. Throughout 

 such a region there is what is called magnetie force, and when certaiu 

 substances are placed in the magnetic field the nuignetic force causes 

 them to become nuignetized by magnetic induction. An effective way 

 of producing a magnetic field is to wind a conducting wire into a coil, 

 and pass a current througli the wire. Within tlie coil we have a region 

 of comparatively strong magnetic force, and when apiece of iron is 

 l)laced there it may be strongly magnetized. Not all substances possess 

 this property. Put a piece of wood or stone or copi)er or silver into 

 the field, and nothing noteworthy happens; but put a piece of irou or 

 ni( ke] oi' cobalt and at once you find that the piece lias become a mag- 

 net. These three metals, with some of tbeir alloys and compounds, 

 stiind out from all other substances in this respect. Not only are they 

 capable of magnetic induction — of becoming magnets while exposed 

 to the action of tiie magnetic field, but when withdrawn from the field 

 they are found to retain a part of the magnetism they acquired. They 

 all show this ])roi»erty of r(^tentiveness, more or less. In some of them 

 this residual nuignetism is feebly held, and may be shaken out or oth- 

 erw isc remo\ed without difficulty. In others, notably in some steels, 

 it is very ]>ersistent, and the 1;ict is taken advantiige of in the manu- 

 facture of permanent magnets, which are simply bars of steel, of proper 

 (juality, which have been subjected to the action of a strong mag- 

 netic field. Of all substances, soft iron is the most suscei)tible io the 

 action of the field. It <-an also under favorable conditions, retain — 

 w hen taken out of the fu'Id — a v^ery large fraction of tlie magnetism that 

 has Ix-en induced — more than nine-tenths, — more indeed than is re- 

 tained by steel; but its hold of this residual magn<'tisin is not firm, and 

 for that reason it will not serve as a material for permanent magnets. 

 My pur])osc to-night is to give some account of the mole(;ular process 

 through which we may conceive magnetic induction to take ])lace. and 

 of the structure which makes residual magnetism possible. 



' Al)8triict of ii Friday eveniiif; (liscouisc delivered at the Royal Iiistitutioii ou 

 Mav 22, 1891. (From Saturn, Oct. I."), 1891; vol. xliv, pp. 566-572.) 



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