uch used. An impedance coil having an open
magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its
external appearance and illustrating particularly the method of
bringing out the terminals of the winding.

[Illustration: Fig. 101. Section of Open-Circuit Impedance Coil]

[Illustration: Fig. 102. Open-Circuit Impedance Coil]

[Illustration: Fig. 103. Closed-Circuit Impedance Coil]

Closed-Circuit:--A type of retardation coil which is largely used in
systems of simultaneous telegraphy and telephony, known as _composite
systems_, is shown in Fig. 103. In the construction of this coil the
core is made of a bundle of fine iron wires first bent into U-shape,
and then after the coils are in place, the free ends of the core are
brought together to form a closed magnetic circuit. The coils have a
large number of turns of rather coarse wire. The conditions
surrounding the use of this coil are those which require very high
impedance and rather large current-carrying capacity, and fortunately
the added requirement, that it shall be placed in a very small space,
does not exist.

Toroidal:--Another type of retardation coil, called the toroidal type
due to the fact that its core is a torus formed by winding a
continuous length of fine iron wire, is shown in diagram in Fig. 104.
The two windings of this coil may be connected in series to form in
effect a single winding, or it may be used as a "split-winding" coil,
the two windings being in series but having some other element, such
as a battery, connected between them in the circuit. Evidently such a
coil, however connected, is well adapted for high impedance, on
account of the low reluctance of its core.

[Illustration: Fig. 104. Symbol of Toroidal Impedance Coil]

This coil is usually mounted on a base-board, the coil being enclosed
in a protecting iron case, as shown in Fig. 105. The terminal wires of
both windings of each coil are brought out to terminal punchings on
one end of the base-board to facilitate the making of the necessary
circuit connections.

[Illustration: Fig. 105. Toroidal Impedance Coil]

The usual diagrammatic symbol for an impedance coil is shown in Fig.
106. This is the same as for an ordinary bar magnet, except that the
parallel lines through the core may be taken as indicating that the
core is laminated, thus conveying the idea of high impedance. The
symbol of Fig. 104 is a good one for the toroidal type of impedance
coil.

[Illust