Magneto Local-Battery Telephone Sets

The magneto is provided with a switch which removes it from the circuit and thus prevents it from introducing a loss when it is not in use. The switch is engaged when the handle is turned. The ringer is of high impedance and is usually connected or "bridged" across the line at all times.

When the receiver is removed from the switch hook, the contacts at the switch hook are closed and direct current flows through the transmitter. When sound waves strike the transmitter diaphragm, a pulsating current as shown in Fig. 4 flows through the primary of the

induction coil and induces an alternating voltage in the secondary circuit. A corresponding alternating current will thus flow through the line, reproducing the sound waves at the distant station.

The induction coil (transformer), which apparently was introduced at about the same time^ref.3 by both Berliner and Edison, greatly improves telephone transmission. In particular: (a) It separates the transmitter and receiver currents so that the direct current for the transmitter does not pass through the receiver, (b) The induction coil may have other than a one-to-one ratio, thus "stepping up" the voltage impressed on the line, or, in other words, the coil matches the impedance of the transmitter to the higher impedance of the line, (c) The use of even a one-to-one ratio repeating coil will greatly increase the percentage change in resistance and thus increase the useful alternating-current component. For instance, if

Figure 3. Local-battery, or magneto, telephone set.

Figure 4. Pulsating current flowing through a transmitter and associated circuit when a transmitter is excited with a pure tone.

the transmitter is in series with a battery, a receiver, and the line, the percentage change in the resistance of the circuit due to the variations in transmitter resistance will be smaller than if the transmitter is connected in series with only the primary of the induction coil and the battery.

A capacitor is often connected as shown by the dotted lines in Fig. 3, especially where a number of local-battery sets are on the same line. This is known as a "sure-ring condenser," and it prevents a set with a receiver off the hook from shunting the ringing currents. Thus, on lines where a number of sets are connected, if one party leaves the receiver off the hook the low impedance of the set will prevent another person from ringing a third party on the line. With a sure-ring condenser, however, the low-frequency ringing currents are not shunted because of the high reactance.