llacies. Such iron
is far better placed either right inside the coils or right outside
them, so that it may properly constitute a part of the magnetic
circuit. The constructions known as Camacho's and Cance's, and one
patented by Mr. S.A. Varley, in 1877, belonging to this delusive order
of ideas, are now entirely obsolete.

Another construction which is periodically brought forward as a
novelty is the use of iron windings of wire or strip in place of
copper winding. The lower electric conductivity of iron, as compared
with copper, makes such a construction wasteful of exciting power. To
apply equal magnetizing power by means of an iron coil implies the
expenditure of about six times as many watts as need be expended if
the coil is of copper.

ELECTROMAGNETS FOR MAXIMUM RANGE OF ATTRACTION.

We have already laid down the principle which will enable us to design
electromagnets to act at a distance. We want our magnet to project, as
it were, its force across the greatest length of air gap. Clearly,
then, such a magnet must have a very large magnetizing power, with
many ampere turns upon it, to be able to make the required number of
magnetic lines pass across the air resistance. Also it is clear that
the poles must not be too close together for its work, otherwise the
magnetic lines at one pole will be likely to curl round and take short
cuts to the other pole. There must be a wider width between the poles
than is desirable in electromagnets for traction.

ELECTROMAGNETS OF MINIMUM WEIGHT.

In designing an apparatus to put on board a boat or a balloon, where
weight is a consideration of primary importance, there is again a
difference. There are three things that come into play--iron, copper,
and electric current. The current weighs nothing, therefore, if you
are going to sacrifice everything else to weight, you may have
comparatively little iron, but you must have enough copper to be able
to carry the electric current; and under such circumstances you must
not mind heating your wires nearly red hot to pass the biggest
possible current. Provide as little copper as you conveniently can,
sacrificing economy in that case to the attainment of your object;
but, of course, you must use fireproof material, such as asbestos, for
insulating, instead of cotton or silk.

A USEFUL GUIDING PRINCIPLE.

In all cases of design there is one leading principle which will be
found of great assistance, namely, that a magnet alway