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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.


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.


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.

The A.S.T.I.C. coil has been developed over the years to produce a most efficient telephone instrument. Today, the induction coil has been reduced in size and refined so that it is now manufactured in an integrated circuit package.

The following was added by Jack Ryan, 1st February 2011.......

I happened to read the Newsletter article on the ATCS WEB site entitled The Induction Coil. The first sentence reads “Almost all telephones manufactured for public exchange use after about 1900, have incorporated an induction coil”. This is true and some manufacturers, for example Automatic Electric, included an induction coil in the base of their 1905 CB candlestick and in the base of the 11 digit “Strowger” automatic candlestick.

What is more interesting is that almost all of the Automatic Electric telephones used on their SxS exchanges between 1908 and about 1915 did not use an induction coil. This includes all the telephones used at Epsom, Official, Geelong and many other installations all over the world. The series (no induction coil) circuit also used an unbiased receiver – also called an electromagnetic or DC receiver.

In their reference books (Automatic Telephony), Smith and Campbell stated that although induction coil automatic telephones existed, their importance was minimal and therefore would not be covered. Also from Automatic Telephony is the following (describing subscriber apparatus):

The (British) GPO Tele No 72 (candlestick) and Tele No 55 (wood wall phone), both made by ATM, also used a series circuit without an induction coil. These were superseded by the Tele No 124 and Tele No 105 in 1921 when the GPO standardised on the induction coil circuit.

Northern Electric (Canada) also made telephones based upon AE patents that used a series circuit. Their N20-UN (pictured right) is an interesting combination of a Western Electric 20-AL, an AE signature stepped base and an N type dial based upon the AE Mercedes dial.

Automatic Electric continued to make series circuit telephones for use on the public network until about 1930. After that time series telephones were restricted to use on PAXs.


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