170 Prof. Tyndall on the Nature of the Force by which 



strength of the iutliiencing magnet. Poisson figured the act 

 of magnetization as consisting of the decomposition of a neutral 

 magnetic 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 cm-rents 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 parallelism, 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 constraint, and when the magnet is removed, 

 the substance returns to its normal condition. Poisson's sepa- 

 rated fluids rush together once more, and Ampere's molecular 

 currents return to their former irregular positions. As our 

 knowledge increases, we shall probably find both hypotheses 

 inadequate to represent the phsenomena ; 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, 

 offer 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 sufiicient 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 nM, the steel being still supposed 

 hard enough to resist magnetization by influence ; the mutual 

 attraction now will be 



nMM', 

 or n times the former attraction ; hence when a variable mag- 



