Let us observe what is the advantage of this arrangement over the case of Fig. 10.  Using the same values of resistance in the transmitter and line, assume the local circuit apart from the transmitter to have a fixed resistance of 5 ohms.  The limits of variations in the local circuit, therefore, are 10 and 55 ohms, thus making the maximum 5.5 times the minimum, or an increase of 450 per cent as against 4.5 per cent in the case of Fig. 10.  The changes, therefore, are 100 times as great.

[Illustration:  Fig. 12.  Conventional Diagram of Talking Circuit]

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.

Detrimental Effects of Capacity.  Electrostatic capacity plays an important part in the transmission of speech.  Its presence between the wires of a line and between them and the earth causes one of the losses from which long-distance telephony suffers.  Its presence in condensers assists in the solution of many circuit and apparatus problems.