Carbon and magneto transmitters differ wholly in their methods of action.  The magneto transmitter produces current; the carbon transmitter controls current.  The former is an alternating-current generator; the latter is a rheostat.  The magneto transmitter produces alternating current without input of any electricity at all; the carbon transmitter merely controls a direct current, supplied by an external source, and varies its amount without changing its direction.

The carbon transmitter, however, may be associated with other devices in a circuit in such a way as to transform direct currents into alternating ones, or it may be used merely to change constant direct currents into undulating ones, which never reverse direction, as alternating currents always do.  These distinctions are important.

[Illustration:  Fig. 10.  Battery in Line Circuit]

Limitations. A carbon transmitter being merely a resistance-varying device, its usefulness depends on how much its resistance can vary in response to motions of air molecules.  A granular-carbon transmitter may vary between resistances of 5 to 50 ohms while transmitting a particular tone, having the lower resistance when its diaphragm is driven inward.  Conceive this transmitter to be in a line as shown in Fig. 10, the line, distant receiver, and battery together having a resistance of 1,000 ohms.  The minimum resistance then is 1,005 ohms and the maximum 1,050 ohms.  The variation is limited to about 4.5 per cent.  The greater the resistance of the line and other elements than the transmitter, the less relative change the transmitter can produce, and the less loudly the distant receiver can speak.

[Illustration:  Fig 11.  Battery in Local Circuit]

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 discussion 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.