Induction Coil

Mr. Edison realized this limitation to the use of the carbon transmitter direct in the line, and contributed the means of removing it. His method is to introduce an induction coil between the line and the transmitter, its function being to translate the variation of the direct current controlled by the transmitter into true alternating currents. An induction coil is merely a transformer, and for the use under discu ion consists of two insulated wires wound around an iron core. Change in the current carried by one of the windings produces a current in the other. If direct current be flowing in one of the windings, and remains constant, no current whatever is produced in the other. It is important to note that it is change, and change only, which produces that alternating current. Fig. 11 shows an induction coil related to a carbon transmitter, a battery, and a receiver. Fig. 12 shows exactly the same arrangement, using conventional signs. The winding of the induction coil which is in series with the transmitter and the battery is called the primary winding; the other is called the secondary winding. In the arrangement of Figs. 11 and 12 the battery has no metallic connection with the line, so that it is called a local battery . The circuit containing the battery, transmitter, and primary winding of the induction coil is called the local circuit . 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, a ume 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. IMG:384091999325875926_fig012_t.gif.png:Illustration_ Fig. 12. Conventional Diagram of Talking Circuit Fig. 12. Conventional Diagram of Talking Circuit View full size illustration. 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 transmi ion meets with but one-quarter of one kind of lo when the sending potential is doubled, one-hundredth of that lo when it is raised tenfold, and similarly. The induction coil, therefore, serves the double purpose of a step-up transformer to limit line lo es 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. IMG:384091999325875926_fig013_t.gif.png:Illustration_ Fig. 13. Induction-Coil Action Fig. 13. Induction-Coil Action View full size illustration. It is by these means that telephone transmitters draw direct current from primary batteries and send high-potential alternating currents over lines; the same proce 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.