and the Maximum of Magnetism of Iron, Steel, and Nickel. 113 



magnetic force ; and the utility of this method of thinking is 

 shown in the method of experimenting adopted for measuring 

 magnetism in absolute measure^ for which I claim that it is 

 the simplest and most accurate of any yet devised. Whether 

 Faraday's theory is correct or not, it is well known that its 

 use will give correct results ; at the present time the tendency 

 of the most advanced thought is toward the theory* ; and indeed 

 it has been pointed out by Sir William Thomson that it follows, 

 from dynamical reasoning upon the magnetic rotation of the 

 plane of polarization of light, that the medium in which this 

 takes place must itself be in rotation, the axis of rotation being 

 in the direction of the lines of force j\ Some substances must 

 of necessity be more capable of assuming this rotary motion than 

 others; and hence arises the notion of magnetic iC conductivity" 

 and "permeability." 



Thomson has pointed out several analogies which may be used 

 in calculating the distribution and direction of the lines of force 

 under various circumstances. He has shown that the mathe- 

 matical treatment of magnetism is the same as that of the flow 

 of heat in a solid, as the static induction of electricity, and as 

 the flow of a frictionless incompressible liquid through a porous 

 solid. It is evident that to these analogies we may add that of 

 the conduction of electricity |. We readily see that the reason 

 of the treatment being the same in each case is that the elemen- 

 tary law of each is similar to Ohm's law. Mr. Webb§ has 

 shown that this law is useful in electrostatics ; and I hope, in a 

 sequel to this paper, to apply it to the distribution of mag- 

 netism : I give two equations derived in this way further on. 



The absolute units to which I have reduced my results are 

 those in which the metre, gramme, and second are the funda- 

 mental units. The unit of magnetizing-ibrce of helix I have 

 taken as that of one turn of wire carrying the unit current per 

 metre of length of helix, and is Am times the unit magnetic 

 field. This is convenient in practice, and also because in the 

 mathematical solution of problems in electrodynamics the mag- 

 netizing-force of a solenoid naturally comes out in this unit. 

 The magnetizing-force of any helix is reduced to this unit by 

 multiplying the strength of current in absolute units by the 

 number of coils in the helix per metre of length. These remarks 



* " On Action at a Distance," Maxwell, ' Nature/ Feb. 27 and March 

 6 and 13, 1873. 



t Thomson's 'Papers on Electricity and Magnetism/ p. 419, note; 

 and Maxwell's ' Treatise on Electricity and Magnetism/ vol. ii. chap. xxi. 



% Maxwell's ' Treatise on Electricity and Magnetism/ arts. 243, 244, 

 and 245. 



§ " Application of Ohm's Law to Problems in Electrostatics/' Phil. 

 Mag. S. 4. vol. xxxv. p. 325 (1868). 



