He also investigated the diamagnetic and paramagnetic properties of substances; and was keenly interested in the phenomena of electrochemical decomposition, accumulating much evidence in favour of Faraday's law and proposing a modified statement of it which was intended to cover certain apparent exceptions.
Those substances which are attracted, or rather which tend, like iron, to move from weaker to stronger parts of the magnetic field, are termed paramagnetic; those which are repelled, or tend to move from stronger to weaker parts of the field, are termed diamagnetic. Between the ferromagnetics and the paramagnetics there is an enormous gap. The maximum magnetic susceptibility of iron is half a million times greater than that of liquid oxygen, one of the strongest paramagnetic substances known.
The permeability of most material substances differs very slightly from unity, being a little greater than I in paramagnetic and a little less in diamagnetic substances.
From the equation K=(µ - I)/47r, it follows that the magnetic susceptibility of a vacuum (where µ = I) is o, that of a diamagnetic substance (where, u I) is positive.
For small bodies other than spheres the coefficient will be different, but its sign will always be negative for diamagnetic substances and positive for others; hence the forces acting on any small body will be in the same directions as in the case of a sphere' Directing Couple acting on an Elongated Body.