et to tell your pupils that it is to be regarded as a
symbol merely,--a symbol, moreover, which is incompetent to cover all
the facts, but which does good practical service whilst we are waiting
for the actual truth. [Footnote: This theory breaks down when applied
to diamagnetic bodies which are repelled by magnets. Like soft iron,
such bodies are thrown into a state of temporary excitement, in virtue
of which they are repelled; but any attempt to explain such a
repulsion by the decomposition of a fluid will demonstrate its own
futility.]

The state of excitement into which iron is thrown by the influence, of
a magnet, is sometimes called 'magnetisation by influence.' More
commonly, however, the magnetism is said to be 'induced' in the iron,
and hence this mode of magnetising is called 'magnetic induction.'
Now, there is nothing theoretically perfect in Nature: there is no
iron so soft as not to possess a certain amount of coercive force, and
no steel so hard as not to be capable, in some degree, of magnetic
induction. The quality of steel is in some measure possessed by iron,
and the quality of iron is shared in some degree by steel. It is in
virtue of this latter fact that the unmagnetised darning-needle was
attracted in your first experiment; and from this you may at once
deduce the consequence that, after the steel has been magnetised, the
repulsive action of a magnet must be always less than its attractive
action. For the repulsion is opposed by the inductive action of the
magnet on the steel, while the attraction is assisted by the same
inductive action. Make this clear to your minds, and verify it by
your experiments. In some cases you can actually make the attraction
due to the temporary magnetism overbalance the repulsion due to the
permanent magnetism, and thus cause two poles of the same kind
apparently to attract each other. When, however, good hard magnets
act on each other from a sufficient distance, the inductive action
practically vanishes, and the repulsion of like poles is sensibly
equal to the attraction of unlike ones.

I dwell thus long on elementary principles, because they are of the
first importance, and it is the temptation of this age of unhealthy
cramming to neglect them. Now follow me a little farther. In
examining the distribution of magnetism in your strip of steel you
raised the needle slowly from bottom to top, and found what we called
a neutral point at the centre.

Now does