471-474] Mechanical Forces 405 



with that arising from the original field, so that the magnetisation of any 

 single particle of the salt in the solution may be regarded as produced 

 entirely by the original field. Hence we have conditions similar to those 

 which obtain electrostatically in a gas. The induced field may be regarded 

 simply as the aggregate of the fields arising from the different particles of 

 the magnetic medium, and is therefore jointly proportional to the density of 

 these particles and to the strength of the inducing field. The latter fact 

 shews that, for a given density of the medium, /A ought to be independent of 

 H, a result to which we shall return later. The former fact shews that, as 

 the density r changes, //, 1 ought to be proportional to r a result analogous 

 to the result that K 1 is proportional to the density in a gas. It has been 

 found experimentally by Quincke * that /z- 1 is approximately proportional 



tO T. 



In gases we have conditions precisely similar to those which obtain when 

 a gas is placed in an electrostatic field. Hence //, 1 must, for a gas, be 

 proportional to r, for exactly the same reason for which K 1 is proportional 

 to T. This result also has been verified by Quincke (. 



Thus we may say that for fluid media, whether liquid or gaseous, //, 1 

 is, in general, proportional to T, where T is the density of the magnetic liquid, 

 in the case of a liquid in solution, or of the gas itself, in the case of a gas. 



473. If we assume the relation 



p-l = cr .............................. (408), 



where c is a constant, we find that expression (407) may be put in the 

 simpler form 



shewing that the whole mechanical force is the same as would be set up by a 

 hydrostatic pressure at every point of the medium of amount ^ H z . 



OTT 



If H varies from point to point of the field, the effect of this pressure will 

 clearly be to urge the medium to congregate in the more intense parts of the 

 field. This has been observed by MatteucciJ for a medium consisting of 

 drops of chloride of iron dissolved in alcohol placed in a medium of olive oil. 

 The drops of solution were observed to move towards the strongest parts of 

 the field. 



Magnetostriction. 



474. If a liquid is placed in a magnetic field, it yields under the 

 influence of the mechanical forces acting upon it, so that we have a 



* Wied. Ann. 24, p. 347. 

 t Wied. Ann. 34, p. 401. 

 J Comptes Rendus, 36, p. 917. 



