The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 98. BISECTED COILS.
133 STANDARD ELECTRICAL DICTIONARY.
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.
134 STANDARD ELECTRICAL DICTIONARY.
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.
135 STANDARD ELECTRICAL DICTIONARY.
The induction coil produces a rapid succession of sparks, which may
spring across an interval of forty inches. The secondary generally ends
in special terminals or electrodes between which the sparking takes
place. A plate of glass, two inches in thickness, can be pierced by
them. In the great Spottiswoode coil there are 280 miles of wire in the
secondary, and the wire is about No. 36 A.W.G.

Fig. 100. VERTICAL SECTION OF INDUCTION COIL.

Fig. l01. PLAN OF INDUCTION COIL CONNECTIONS.
Induction coils have quite extended use in electrical work. They are
used in telephone transmitters, their primary being in circuit with the
microphone, and their secondary with the line and receiving telephone.
In electric welding, and in the alternating current system they have
extended application. In all these cases they have no automatic circuit
breaker, the actuating current being of intermittent or alternating
type.
136 STANDARD ELECTRICAL DICTIONARY.

In the cuts the general construction of an induction coil is shown. In
the sectional elevation, Fig. 100, A, is the iron core; B is the primary
of coarse wire; C is a separating tube, which may be of pasteboard; D is
the secondary of fine wire; E, E are the binding posts connected to the
secondary; H, H are the heads or standards; K, K are the terminals of
the primary; F is the vibrating contact spring; G, a standard carrying
the contact screw; J is the condenser with wires, L, M, leading to it.
Referring to the plan, Fig. 101, H represents the primary coil; B and A
are two of the separate sheets of the condenser, each sheet with
projecting ears; G, G are the heads of the coil; the dark lines are
connections to the condenser. One set of sheets connects with the
primary coil at C, and also with the vibrating spring shown in plan and
in the elevation at F. The other set of sheets connects with the post,
carrying the contact screw. The other terminal of the primary runs to a
binding post E. F, in the plan is a binding post in connection with the
standard and contact screw.
Coil, Induction, Inverted.
An induction coil arranged to have a lower electro-motive force in the
secondary than in the primary. This is effected by having more
convolutions in the primary wire than in the secondary. Such coils in
practice are used with the alternating current and then do not include a
circuit breaker or condenser. They are employed in alternating current
system and in electric welding. (See Welding, Electric--Converter.)
In the cut an inverted coil, as constructed for electric welding is
shown. In it the primary coil is marked P; the secondary, merely a bar
of metal, is marked E, with terminals S, S; the heavy coils, I, of iron
wire are the core; K is a screw for regulating the clamps; J, Z is a
second one for the same purpose, while between D and D' the heat is
produced for welding the bars, B, B', held in the clamps, C, C'. It will
be seen how great may be the difference in turns between the single
circle of heavy copper rod or bar which is the secondary of the coil,
and the long coil of wire forming the primary.

Fig. 102. INVERTED INDUCTION COIL FOR ELECTRIC WELDING.
137 STANDARD ELECTRICAL DICTIONARY.
Coil, Induction, Telephone.
An induction coil used in telephone circuits. It is placed in the box or
case near the transmitter. The primary is in circuit with the
microphone. The secondary is in circuit with the line and receiving
telephone. In the Bell telephone apparatus the primary of the induction
coil is wound with No. 18 to 24 A. W. G. wire to a resistance of 1/2
ohm; the secondary, with No. 36 wire to a resistance of 80 ohms. The
Edison telephone induction coil was wound with similar wires to a
resistance of 3 to 4 ohms and of 250 ohms respectively.
Coil, Magnetizing.
A coil of insulated wire for making magnets; and for experimental uses;
it has a short axis and central aperture of as small size as consistent
with the diameter of the bar to be magnetized, which has to pass through
it readily. The wire may be quite heavy, 2 or 3 millimeters (.08--.12
inch) thick, and is cemented together with carpenter's glue, or with
shellac or ethereal solution of gum copal. In use it is passed over the
bar a few times while a heavy current is going through it. It is used
for magic circles also. (See Circle, Magic.)