shown in Fig. 8, which is reproduced from the actual figure made by iron
filings. To magnetize the iron bar of the electro-magnet as strongly as
possible the wire should be coiled many times round, and the current
should be as strong as possible. This mode of making an iron rod or bar
into a powerful magnet is adopted in every dynamo-electric machine. For,
as will be presently explained, very powerful magnets are required, and
these magnets are most effectively made by sending the electric currents
through spiral coils of wire wound (as in Fig. 8) round the bars that
are to be made into magnets.

[Illustration: Fig. 8]

The reader will at this point probably be ready to jump to the
conclusion that magnets and currents are alike surrounded by a sort of
magnetic atmosphere, and such a view may help those to whom the subject
is fresh to realize how such actions as we have been describing can be
communicated from one magnet to another, or from a current to a magnet.
Nevertheless such a conclusion would be both premature and inaccurate.
Even in the most perfect vacuum these actions still go on, and the lines
of force can still be traced. It is probably more correct to conclude
that these magnetic actions are propagated through space not by special
magnetic atmospheres, but by there being movements and pressures and
tensions in the _ether_ which is believed to pervade all space as a
very thin medium more attenuated than the lightest gas, and which when
subjected to electro-magnetic forces assumes a peculiar state, and
gives rise to the actions which have been detailed in the preceding
paragraphs.

[Illustration: Fig. 9.]