Almost all telephones manufactured for public exchange use after about 1900, have incorporated an induction coil. The basic functions of an induction coil, in a magneto or local battery telephone, are as follows:

1. To enable the transmitter to operate in a low impedance circuit. This reduces the battery voltage required to supply the minimum DC current through the transmitter for satisfactory operation. The normal arrangement is two cells each of 1.5 volts, the 1ohm winding of the induction coil resulting in at least 50mA through the transmitter.
2. To prevent DC from the local battery flowing through the receiver which would reduce its efficiency.
3. To match the transmitter impedance (about 60 ohms) with that of the line and distant telephone (about 600 ohms) resulting in higher efficiency.

Sidetone.

When sounds picked up by the transmitter are reproduced in the local receiver, the effect is called "sidetone". In early magneto telephones, all the transmitted speech passed through and was heard in the local receiver.

Excessive sidetone has two disadvantages:

1. When the speaker hears his own voice too loudly in the local receiver, he tends to lower his voice. This results in the distant party having difficulty hearing what is said. (Conversely, no sidetone in the local receiver causes the speaker to shout, thinking the telephone is not working).
2. Background noises tend to "mask" the received speech making it difficult to hear and understand what is being said by the distant party.

Early transmitters and receivers were relatively inefficient and therefore did not produce excessive sidetone. People usually had to shout to be heard on most calls. As more efficient equipment entered the market, a means was required to control the effects of sidetone.

A.S.T.I.C.

Enter the Anti-Sidetone Induction Coil, abbreviated to A.S.T.I.C. The principle of operation is similar to a Wheatstone Bridge. By providing an impedance in the telephone to balance the line to the distant telephone, then the receiver, connected across the midpoint of the bridge, will receive only a small amount of the transmitted signal. Due to the impedances of the coil, little of the incoming speech will be lost and the signal will be heard at maximum volume.