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.)
Fig. 103. MAGNETIZING COIL.
Coil, Resistance.
A coil constructed for the purpose of offering a certain resistance to a
steady current. This resistance may be for the purpose of carrying out
quantitative tests, as in Wheatstone bridge work (see Wheatstone's
Bridge), or simply to reduce the intensity of a current. For the first
class of work the coils are wound so as to prevent the creation of a
magnetic field. This is effected by first doubling the wire without
breaking it, and then starting at the bend the doubled wire, which is
insulated, is wound on a bobbin or otherwise until a proper resistance
is shown by actual measurement. The coils are generally contained or set
in closed boxes with ebonite tops. Blocks of brass are placed on the
top, and one end from one coil and one end from the next connect with
the same block. By inserting a plug, P, so as to connect any two blocks,
which have grooves reamed out for the purpose, the coil beneath will be
short circuited. German silver, platinoid or other alloy, q. v., is
generally the material of the wire. A great object is to have a wire
whose resistance will be unaffected by heat.