Types. Open-Circuit:—­Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much 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.

[Illustration:  Fig. 106.  Symbol of Impedance Coil]

Induction Coil.  An induction coil consists of two or more windings of wire interlinked by a common magnetic circuit.  In an induction coil having two windings, any change in the strength of the current flowing in one of the windings, called the primary, will cause corresponding changes in the magnetic flux threading the magnetic circuit, and, therefore, changes in flux through the other winding, called the secondary.  This, by the laws of electromagnetic induction, will produce corresponding electromotive forces in the secondary winding and, therefore, corresponding currents in that winding if its circuit be closed.