of soft iron wires. The
use of laminated cores is for the purpose of preventing eddy currents,
which, if allowed to flow, would not only be wasteful of energy but
would also tend to defeat the desired high impedance. Sometimes in
iron-clad impedance coils, the iron shell is slotted longitudinally to
break up the flow of eddy currents in the shell.

Frequently electromagnetic coils have only the function of offering
impedance, where no requirements exist for converting any part of the
electric energy into mechanical work. Where this is the case, such
coils are termed _impedance_, or _retardation_, or _choke coils_,
since they are employed to impede or to retard or to choke back the
flow of rapidly varying current. The distinction, therefore, between
an impedance coil and the coil of an ordinary electromagnet is one of
function, since structurally they may be the same, and the same
principles of design and construction apply largely to each.

_Number of Turns_. It should be remembered that an impedance coil
obstructs the passage of fluctuating current, not so much by ohmic
resistance as by offering an opposing or counter-electromotive force.
Other things being equal, the counter-electromotive force of
self-induction increases directly as the number of turns on a coil and
directly as the number of lines of force threading the coil, and this
latter factor depends also on the reluctance of the magnetic circuit.
Therefore, to secure high impedance we need many turns or low
reluctance, or both. Often, owing to requirements for direct-current
carrying capacity and limitations of space, a very large number of
turns is not permissible, in which case sufficiently high impedance to
such rapid fluctuations as those of voice currents may be had by
employing a magnetic circuit of very low reluctance, usually a
completely closed circuit.

_Kind of Iron. _An important factor in the design of impedance coils
is the grade of iron used in the magnetic circuit. Obviously, it
should be of the highest permeability and, furthermore, there should
be ample cross-section of core to prevent even an approach to
saturation. The iron should, if possible, be worked at that density of
magnetization at which it has the highest permeability in order to
obtain the maximum impedance effects.

_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 m