The relation between the windings of the induction
The relation between the windings of the induction coil in this
practice are such that the secondary winding contains many more turns
than the primary winding. Changes in the circuit of the primary
winding produce potentials in the secondary winding correspondingly
higher than the potentials producing them. These secondary potentials
depend upon the _ratio_ of turns in the two windings and therefore,
within close limits, may be chosen as wished. High potentials in the
secondary winding are admirably adapted to transmit currents in a
high-resistance line, for exactly the same reason that long-distance
power transmission meets with but one-quarter of one kind of loss when
the sending potential is doubled, one-hundredth of that loss when it
is raised tenfold, and similarly. The induction coil, therefore,
serves the double purpose of a step-up transformer to limit line
losses and a device for vastly increasing the range of change in the
transmitter circuit.
Fig. 13 is offered to remind the student of the action of an induction
coil or transformer in whose primary circuit a direct current is
increased and decreased. An increase of current in the local winding
produces an impulse of _opposite_ direction in the turns of the
secondary winding; a decrease of current in the local winding produces
an impulse of _the same_ direction in the turns of the secondary
winding. The key of Fig. 13 being closed, current flows upward in the
primary winding as drawn in the figure, inducing a downward impulse of
current in the secondary winding and its circuit as noted at the right
of the figure. On the key being opened, current ceases in the primary
circuit, inducing an upward impulse of current in the secondary
winding and circuit as shown. During other than instants of opening
and closing (changing) the local circuit, no current whatever flows in
the secondary circuit.
[Illustration: Fig. 13. Induction-Coil Action]
It is by these means that telephone transmitters draw direct current
from primary batteries and send high-potential alternating currents
over lines; the same process produces what in Therapeutics are called
“Faradic currents,” and enables also a simple vibrating contact-maker
to produce alternating currents for operating polarized ringers of
telephone sets.