154 ON MAGNETISM. 



70. Defect of this theory : sketch of Poissons more 

 complete theory. 



We abstain, in this article, from expressing any 

 doubt of the correctness of the assumption from which 

 the preceding investigation starts, namely, that by the 

 action of external magnetic force, every particle of the 

 iron, or a limited proportion of the iron in every small 

 space, is converted into a small magnet, whose axis is 

 parallel to the axis of the external magnetism. The 

 effect of such external magnetic force has been duly 

 taken into account. But there is another disturbing 

 magnetism not taken into account, namely, that every 

 small magnet thus produced in the mass of iron pro- 

 duces a disturbing effect on every other small magnet. 



We have reason to think, from the magnitude of 

 that phenomenon of induction whose effects are most 

 accurately known, namely the quadrantal disturbance, 

 that the total effect of the internal action of the minute 

 magnets is small in comparison with the effect of the 

 terrestrial force : in some instances perhaps one-fifteenth 

 part (with wide uncertainty). If, however, this be 

 considered as a fair representation of its magnitude, 

 then none of the preceding conclusions can be very 

 wrong, and the theory of the last article may be accepted 

 as sufficient for all practical cases. And it has this 

 merit, that it shews clearly the dependence of the mag- 

 nitude and sign of each force upon the distribution of 

 the masses of iron. 



Poisson undertook, in the Memoires de VInstitut de 



