those frequent cases where it is necessary to cause the line to return
over a path that it has already traversed, it is more advantageous to
effect the return through the earth or to utilize the street water mains
or gas pipes as conductors. This return arrangement may likewise be
applied to the lines of secondary, tertiary, etc., order, as may easily be
seen.

The induction is effected by the aid of bobbins whose interior consists of
a bundle of soft iron. The wire of the inducting current is wound directly
around this core. The wire of the induced current is superposed upon the
first and presents a large number of spirals. It is useless to say that
these wires must be perfectly insulated from each other, as well as from
the soft iron core. We shall call primary bobbins those which are
interposed in the principal line, and secondary bobbins those in which the
inducting current is a secondary one, and so on.

It will be at once seen that this arrangement permits of continuing the
distribution of electricity to the interior of buildings by the simple
adjunction of one or several bobbins. Each electric apparatus, whether it
be a lamp or other mechanism, is furnished with a special current. If the
number of these apparatus be increased, it is only necessary to increase
the number of bobbins in the same ratio, on condition, be it understood,
that the intensity of the currents remain sufficient to secure a proper
working of the apparatus in question. When such intensity diminishes to
too great a degree, the bobbin must be replaced by a stronger one.

[Illustration: DISTRIBUTION OF ELECTRICITY BY INDUCTION.]

It results from what precedes that each apparatus must be put in in such a
way as to permit, of the opening and closing of the corresponding circuit.