These have already been dealt with in the preceding lecture; the
characteristic feature of all the forms suitable for traction being
the compact magnetic circuit.
Several times it has been proposed to increase the power of
electromagnets by constructing them with intermediate masses of iron
between the central core and the outside, between the layers of
windings. All these constructions are founded on fallacies. 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.