Coils, Bisected.
Resistance coils with connections at their centers, as shown in the
diagram. They are used for comparing the resistances of two conductors.
The connections are arranged as shown in the coil, each coil being
bisected. For the wires, movable knife-edge contacts are employed. The
principle of the Wheatstone bridge is used in the method and
calculations.
Coil, Earth.
A coil of wire mounted with commutator to be rotated so as to cut the
lines of force of the earth's magnetic field, thereby generating
potential difference. The axis of rotation may be horizontal, when the
potential will be due to the vertical component of the earth's field, or
the axis may be horizontal, when the potential will be due to the
vertical component, or it may be set at an intermediate angle.
Synonym--Delezenne's Circle.
Fig. 99. DELEZENNE'S CIRCLE OR EARTH COIL.
Coil, Electric.
A coil of wire used to establish a magnetic field by passing a current
through it. The wire is either insulated, or so spaced that its
convolutions do not touch.
Coil, Flat.
A coil whose windings all lie in one plane, making a sort of disc, or an
incomplete or perforated disc.
Coil, Induction.
A coil in which by mutual induction the electromotive force of a portion
of a circuit is made to produce higher or lower electro-motive force, in
an adjoining circuit, or in a circuit, part of which adjoins the
original circuit, or adjoins part of it.
An induction coil comprises three principal parts, the core, the primary
coil and the secondary coil. If it is to be operated by a steady
current, means must be provided for varying it or opening and closing
the primary circuit. A typical coil will be described.