ect in a given circuit arrangement, and from the knowledge so
gained to substitute another just fitted to the conditions. It is in
such a substitution that the table is of most value. Assume a case in
which are required a spool and core of a given size with a winding of,
say No. 25 single silk-covered wire, of a resistance of 50 ohms.
Assume also that the circuit regulations required that this spool
should be rewound so as to have a resistance of, say 1,000 ohms. What
size single silk-covered wire shall be used? Manifestly, the winding
space remains the same, or nearly so. The resistance is to be
increased from 50 to 1,000 ohms, or twenty times its first value.
Therefore, the wire to be used must show in the table twenty times as
many ohms per cubic inch as are shown in No. 25, the known first size.
This amount would be twenty times 7.489, which is 149.8, but there is
no size giving this exact resistance. No. 32, however, is very nearly
of that resistance and if wound to exactly the same depth would give
about 970 ohms. A few turns more would provide the additional thirty
ohms.

Similarly, in a coil known to possess a certain number of turns, the
table will give the size to be selected for rewinding to a greater or
smaller number of turns. In this case, as in the case of substituting
a winding of different resistance, it is unnecessary to measure and
calculate upon the dimensions of the spool and core. Assume a spool
wound with No. 30 double silk-covered wire, which requires to be
wound with a size to double the number of turns. The exact size to do
this would have 8922. turns per square inch and would be between No.
34 and No. 35. A choice of these two wires may be made, using an
increased winding depth with the smaller wire and a shallower winding
depth for the larger wire.

Impedance Coils. In telephony electromagnets frequently serve, as
already stated, to perform other functions than the producing of
motion by attracting or releasing their armatures. They are required
to act as impedance coils to present a barrier to the passage of
alternating or other rapidly fluctuating currents, and at the same
time to allow the comparatively free passage of steady currents. Where
it is desired that an electromagnet coil shall possess high impedance,
it is usual to employ a laminated instead of a solid core. This is
done by building up a core of suitable size by laying together thin
sheets of soft iron, or by forming a bundle