PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
15 
its magnetization varies with the strength of the influencing magnet. Poisson 
figured the act of magnetization as consisting of the decomposition of a neutral mag- 
netic fluid into north and south magnetism, the amount of the decomposition being 
proportional to the strength of the magnet which produces it. Ampere, discarding 
the notion of magnetic fluids, figured the molecules of soft iron as surrounded by 
currents of electricity, and conceived the act of magnetization to consist in setting 
the planes of these molecular currents parallel to each other : the degree of paral- 
lelism, or in other words, the intensity of the magnetization, depending, as in Poisson’s 
hypothesis, upon the strength of the influencing magnet. 
The state into which the soft iron is here supposed to be thrown is a state of con- 
straint, and when the magnet is removed, the substance returns to its normal condi- 
tion. Poisson’s separated fluids rush together once more, and Ampere’s molecular 
currents return to their former irregular positions. As our knowledge increases, w’-e 
shall probably find both hypotheses inadequate to represent the phenomena; the only 
thing certain is, that the soft iron, when acted upon by the magnet, is thrown into 
an unusual condition, in virtue of which it is attracted ; and that the intensity of this 
condition is a function of the force which produces it. 
There are, however, certain bodies which, unlike soft iron, oflfer a great resistance 
to the imposition of the magnetic state, but when once they are magnetized they do 
not, on the removal of the magnet, return to their neutral condition, but on the 
contrary retain the magnetism impressed on them. It is in virtue of this quality that 
steel can be formed into compass needles and permanent magnets. This power of 
resistance and retention is named by Poisson coercive force. 
Let us conceive a body already magnetized, and in which coercive force exists in 
a very high degree — a piece of very hard steel for example — to be brought near a 
magnet, the strength of which is not sufficient to magnetize the steel further. To 
simplify the matter let us fix our attention upon the south pole of the magnet, and 
conceive it to act upon the north pole of the piece of steel. Let the magnetism of 
the said south pole, referred to any unit, be M, and of the north pole of the steel, M'; 
then their mutual attraction, at the unit of distance, is expressed by the product 
MM'. Conceive now the magnet to increase in power from M to wM, the steel being 
still supposed hard enough to resist magnetization by influence ; the mutual attrac- 
tion now will be 
wMM', 
or n times the former attraction ; hence when a variable magnetic pole acts on an 
opposite one of constant power, the attraction is proportional to the strength of the 
former. 
Let us now take a body whose magnetization varies with that of the magnet : a south 
pole of the strength M induces in such a body a north pole of the strength M', and 
the attraction which results from their mutual action is 
MM'. 
