The core is a mass of soft iron preferably divided to prevent extensive
Foucault currents. A cylindrical bundle of soft iron wires is generally
used. Upon this the primary coil of reasonably heavy wire, and of one or
two layers in depth, is wrapped, all being carefully insulated with
shellac and paper where necessary. The secondary coil is wrapped upon or
over the primary. It consists of very fine wire; No. 30 to 36 is about
the ordinary range. A great many turns of this are made. In general
terms the electro-motive force developed by the secondary stands to that
of the primary terminals in the ratio of the windings. This is only
approximate.
The greatest care is required in the insulating. The secondary is
sometimes wound in sections so as to keep those parts differing greatly
in potential far from each other. This prevents sparking, which would
destroy the insulation.
A make and break, often of the hammer and anvil type, is operated by the
coil. (See Circuit Breaker, Automatic.) As the current passes through
the primary it magnetizes the core. This attracts a little hammer which
normally resting on an anvil completes the circuit. The hammer as
attracted is lifted from the anvil and breaks the circuit. The soft iron
core at once parts with its magnetism and the hammer falls upon the
anvil again completing the circuit. This operation goes on rapidly, the
circuit being opened and closed in quick succession.
Every closing of the primary circuit tends to produce a reverse current
in the secondary, and every opening of the primary circuit tends to
produce a direct current in the secondary. Both are of extremely short
duration, and the potential difference of the two terminals of the
secondary may be very high if there are many times more turns in the
secondary than in the primary.
The extra currents interfere with the action of an induction coil. To
avoid their interference a condenser is used. This consists of two
series of sheets of tin foil. Leaves of paper alternate with the sheets
of tin-foil, the whole being built up into a little book. Each sheet of
tin-foil connects electrically with the sheet next but one to it. Thus
each leaf of a set is in connection with all others of the same set, but
is insulated from the others. One set of leaves of tin-foil connects
with the hammer, the other with the anvil. In large coils there may be
75 square feet of tin-foil in the condenser.
The action of the condenser is to dispose of the direct extra current.
When the primary circuit is opened this current passes into the
condenser, which at once discharges itself in the other direction
through the coil. This demagnetizes the core, and the action intensifies
and shortens the induced current. The condenser prevents sparking, and
in general improves the action of the coil.
Many details enter into the construction of coils, and many variations
in their construction obtain. Thus a mercury cup into which a plunger
dips often replaces the anvil and hammer.