MAGNETIC PEEMEABILITY OF NICKEL AND COBALT 57 



According to the first theory, the magnetization of the iron is repre- 

 sented by the excess of the action of the electromagnet over that of the 

 coil alone; while by the second, when the coil ia very close around the 

 iron, the whole action is due to the magnetization of the iron. The 

 natural unit of magnetism to be used in the first theory is that quantity 

 which will repel an equal quantity at a unit's distance with a unit of 

 force; on the second it is the number of lines of force which pass 

 through a unit of surface when that surface is placed in a unit field 

 perpendicular to the lines of force. The first unit is 4?r times the 

 second. Now when a magnetic force of intensity & 1 acts upon a mag- 

 netic substance, we shall have 33 = +4-$, in which 33 is the mag- 

 netization of the substance according to Faraday's theory, and is what 

 I formerly called the magnetic field, but which I shall hereafter call, 

 after Professor Maxwell, the magnetic induction. % is the intensity 

 of magnetization according to the German theory, expressed in terms 

 of the magnetic moment of the unit of volume. Now, when the sub- 

 stance is in the shape of an infinitely long rod placed in a magnetic field 



01 



parallel to the lines of force, the ratio 2 ==// is called the magnetic 







permeability of the substance, and the ratio = K is Neumann's co- 

 efficient of magnetization by induction. Now experiment shows that 

 for large values of Q the values of both n and K decrease, so that 

 we may expect either $ or both 33 and % to attain a maximum value. 

 In my former paper I assumed that 33 as well as $ attain a maxi- 

 mum; but on further considering the subject I see that we have no data 

 for determining which it is at present. If it were possible for 53 to 

 attain a maximum value so that // should approach to 0, K would be 

 negative, and the substance would then become diamagnetic for very 

 high magnetizing forces. 2 This is not contrary to observation; for at 

 present we lack the means of producing a sufficiently intense magnetic 

 field to test this experimentally, at least in the case of iron. To pro- 

 duce this effect at ordinary temperatures, we must have a magnetic field 

 greater than the following for iron 175,000, for nickel 63,500, and for 



1 1 shall hereafter in all my papers use the notation as given in Professor Maxwell's 

 ' Treatise on Electricity and Magnetism ;' for comparison with my former paper I 

 give the following: 



33 in this paper = Q in former one. 



6 " = 4;rM " 



3 " =-M 



'See Maxwell's 'Treatise on Electricity and Magnetism,' art. 844. J. C. M. 



