One of the most common applications of the repeating coil in telephone work is in connection with the common battery cord circuit, as illustrated by Figure 17-7. Here the alternating current flow in one subscriber's line is repeated into the other subscriber's line with little energy loss, and at the same time the windings of the coils afford the proper direct-current connections for each subscriber's station to receive a superimposed d-c current for transmitter supply. Another very general use of repeating coils in the telephone plant is for deriving "phantom" circuits. Here the coils serve a unique purpose which has no counterpart in electric power work, and is not included in the classification of transformer functions given above. We shall therefore need to consider this application more fully. However, it may be noted that the coils, while serving this particular purpose, may also function as impedance matching devices.
The repeating coils employed at the terminals are designed for voice-current and ringing-current frequencies, and do not appreciably impair transmission over the principal or side circuits. The third or phantom circuit is formed by connecting to the middle points of the line sides of the repeating coil windings, as shown in the Figure. Since the two wires of each side circuit are identical, any current set up in the phantom circuit will equally divide at the mid-point of the repeating coil line windings. One part of the current will flow through one-half of the line winding, and the other part of the current will flow in the opposite direction through the other half of the line winding. The inductive effects will be neutralized, and there will be no resultant current set up in the drop or switchboard side of the repeating coil. Since the phantom current divides into two equal parts, the halves will flow in the same direction through the respective conductors of one side circuit, and likewise return in the other side circuit. At any one point along a side circuit, there will be no difference of potential between the two wires due to current in the phantom circuit, and a telephone receiver bridged across them will not detect the phantom conversation.
Since there is no connection, inductive or otherwise, between the two circuits at the terminals, it is equally true that a conversation over a side circuit cannot be heard in th ephantom. This can be understood by imagining a flow in the closed side circuit through the line wires and the windings of the repeating coils at each end. With the side circuit conductors electrically equal, there can be no difference of potential between the mid-point of the repeating coil winding at one end and the mid-point of the repeating coil line winding at the other end because the drops of potential for the two parts of the side circuit are equal and opposite. If the side circuit, therefore, impresses no difference of potential on any part of the phantom circuit, the side circuit conversation cannot be heard over the phantom.
In the theory of the phantom it should not be forgotten that the conductors are assumed to be electrically identical, or in other words, the conductors are perfectly "balanced". The phantom is very sensitive to the slightest upset of this balance, and circuits that are sufficiently balanced to prevent objectionable crosstalk or noise in physical circuit operation, may not be sufficiently balanced for successful phantom operation.
The 62- and 93-type coils have toroidal cores made of many turns of fine-gage silicon-steel wire sawed through at one point to introduce a gap in the magnetic circuit. In the 93-type coil this gap is filled with compressed powdered iron which, while increasing slightly the reluctance of the core gives it a high degree of magnetic stability, preventing permanent magnetization under abnormal service conditions. In the 62-type coil the gap in the magnetic circuit is unfilled which tends to make the coil even more stable. This coil is especially well adapted for use on circuits composited for d-c telegraph operation. The same feature, however, tends to make the 62 series inefficient at low frequencies and they cannot be used on circuits employing 20-cycle signalling, whereas the 93 series may be used for such purposes. Standard 173-type coils are built with permalloy cores.
STANDARD REPEATING COILS
LINE TO DROP 4-3 AND 8-7
TO 2-1 AND 6-5
Telephone and Telegraph Work
A Training Course Text
Prepared for Employees of the
Long Lines Department
AMERICAN TELEPHONE AND TELEGRAPH COMPANY