Volume I Part 18 (1/2)
_Bridging Battery with Impedance Coils._ The method of feeding current to the line from the common battery, shown in Fig. 130, is called the ”split repeating-coil” method. As distinguished from this is the impedance-coil method which is shown in Fig. 131. In this the battery is bridged across the circuit of the combined lines in series with two impedance coils, _1_ and _2_, one on each side of the battery. The steady currents from the battery find ready path through these impedance coils which are of comparatively low ohmic resistance, and the current divides and pa.s.ses in multiple over the circuits of the two lines. Voice currents, however, originating at either one of the stations, will not pa.s.s through the shunt across the line at the central office on account of the high impedance offered by these coils, and as a result they are compelled to pa.s.s on to the distant station and affect the receiver there, as desired.
This impedance-coil method seems to present the advantage of greater simplicity over the repeating-coil method shown in Fig. 130, and so far as talking efficiency is concerned, there is little to choose between the two. The repeating-coil method, however, has the advantage over this impedance-coil method, because by it the two lines are practically divided except by the inductive connection between the two windings, and as a result an unbalanced condition of one of the connected lines is not as likely to produce an unbalanced condition in the other as where the two lines are connected straight through, as with the impedance-coil method. The substation arrangement of Fig. 131 is the same as that of Fig. 130.
[Ill.u.s.tration: Fig. 132. Double-Battery Kellogg System]
_Double Battery with Impedance Coils._ A modification of the impedance-coil method is used in all of the central-office work of the Kellogg Switchboard and Supply Company. This employs a combination of impedance coils and condensers, and in effect isolates the lines conductively from each other as completely as the repeating-coil method. It is characteristic of all the Kellogg common-battery systems that they employ two batteries instead of one, one of these being connected in all cases with the calling line of a pair of connected lines and the other in all cases with the called line. As shown in Fig. 132, the left-hand battery is connected with the line leading to Station A through the impedance coils _1_ and _2_. Likewise, the right-hand battery is connected to the line of Station B through the impedance coils _3_ and _4_. These four impedance coils are wound on separate cores and do not have any inductive relation whatsoever with each other. Condensers _5_ and _6_ are employed to completely isolate the lines conductively. Current from the left-hand battery, therefore, pa.s.ses only to Station A, and current from the right-hand battery to Station B. Whenever the transmitter at Station A is actuated the undulations of current which it produces in the line cause a varying difference of potential across the outside terminals of the two impedance coils _1_ and _2_. This means that the two left-hand terminals of condensers _5_ and _6_ are subjected to a varying difference of potential and these, of course, by electrostatic induction, cause the right-hand terminals of these condensers to be subject to a correspondingly varying difference of potential. From this it follows that alternating currents will be impressed upon the right-hand line and these will affect the receiver at Station B.
A rough way of expressing the action of this circuit is to consider it in the same light as that of the impedance-coil circuit shown in Fig.
131, and to consider that the voice currents originating in one line are prevented from pa.s.sing through the bridge paths at the central office on account of the impedance, and are, therefore, forced to continue on the line, being allowed to pa.s.s readily by the condensers in series between the two lines.
_Kellogg Substation Arrangement._ An interesting form of substation circuit which is employed by the Kellogg Company in all of its common-battery telephones is shown in Fig. 132. In pa.s.sing, it may be well to state that almost any of the substation circuits shown in this chapter are capable of working with any of the central-office circuits. The different ones are shown for the purpose of giving a knowledge of the various substation circuits that are employed, and, as far as possible, to a.s.sociate them with the particular central-office arrangements with which they are commonly used.
In this Kellogg substation arrangement the line circuit pa.s.ses first through the transmitter and then divides, one branch pa.s.sing through an impedance coil _7_ and the other through the receiver and the condenser _8_, in series. The steady current from the central-office battery finds ready path through the transmitter and the impedance coil, but is prevented from pa.s.sing through the receiver by the barrier set up by the condenser _8_. Voice currents, however, coming over the line to the station, find ready path through the receiver and the condenser but are barred from pa.s.sing through the impedance coil by virtue of its high impedance.
In considering the action of the station as a transmitting station, the variations set up by the transmitter pa.s.s through the condenser and the receiver at the same station, while the steady current which supplies the transmitter pa.s.ses through the impedance coil. Impedance coils used for this purpose are made of low ohmic resistance but of a comparatively great number of turns, and, therefore, present a good path for steady currents and a difficult path for voice currents. This divided circuit arrangement employed by the Kellogg Company is one of the very simple ways of eliminating direct currents from the receiver path, at the same time allowing the free pa.s.sage of voice currents.
[Ill.u.s.tration: Fig. 133. Dean System]
_Dean Substation Arrangement._ In marked contrast to the scheme for keeping steady current out of the receiver circuit employed by the Kellogg Company, is that shown in Fig. 133, which has been largely used by the Dean Electric Company, of Elyria, Ohio. The central-office arrangement in this case is that using the split repeating coil, which needs no further description. The substation arrangement, however, is unique and is a beautiful example of what can be done in the way of preventing a flow of current through a path without in any way insulating that path or placing any barrier in the way of the current.
It is an example of the prevention of the direct flow of current through the receiver by so arranging the circuits that there will always be an equal potential on each side of it, and, therefore, no tendency for current to flow through it.
In this substation arrangement four coils of wire--_1_, _2_, _3_, and _4_--are so arranged as to be connected in the circuit of the line, two in series and two in multiple. The current flowing from the battery at the central office, after pa.s.sing through the transmitter, divides between the two paths containing, respectively, the coils _1_ and _3_ and the coils _2_ and _4_. The receiver is connected between the junction of the coils _2_ and _4_ and that of _1_ and _3_. The resistances of the coils are so chosen that the drop of potential through the coil _2_ will be equal to that through the coil _1_, and likewise that through the coil _4_ will be equal to that through the coil _3_. As a result, the receiver will be connected between two points of equal potential, and no direct current will flow through it.
How, then, do voice currents find their way through the receiver, as they evidently must, if the circuit is to fulfill any useful function?
The coils _2_ and _3_ are made to have high impedance, while _1_ and _4_ are so wound as to be non-inductive and, therefore, offer no impedance save that of their ohmic resistance. What is true, therefore, of direct currents does not hold for voice currents, and as a result, the voice currents, instead of taking the divided path which the direct currents pursued, are debarred from the coils _2_ and _3_ by their high impedance and thus pa.s.s through the non-inductive coil _1_, the receiver, and the non-inductive coil _4_.
This circuit employs a Wheatstone-bridge arrangement, adjusted to a state of balance with respect to direct currents, such currents being excluded from the receiver, not because the receiver circuit is in any sense opaque to such direct currents, but because there is no difference of potential between the terminals of the receiver circuit, and, therefore, no tendency for current to flow through the receiver.
In order that fluctuating currents may not, for the same reason, be caused to pa.s.s by, rather than through, the receiver circuit, the diametrically-opposed arms of the Wheatstone bridge are made to possess, in large degree, self-induction, thereby giving these two arms a high impedance to fluctuating currents. The conditions which exist for direct currents do not, therefore, exist for fluctuating currents, and it is this distinction which allows alternating currents to pa.s.s through the receiver and at the same time excludes direct currents therefrom.
In practice, the coils _1_, _2_, _3_, and _4_ of the Dean substation circuit are wound on the same core, but coils _1_ and _4_--the non-inductive ones--are wound by doubling the wire back on itself so as to neutralize their self-induction.
_Stromberg-Carlson._ Another modification of the central-office arrangement and also of the subscribers' station circuits, is shown in Fig. 134, this being a simplified representation of the circuits commonly employed by the Stromberg-Carlson Telephone Manufacturing Company. The battery feed at the central office differs only from that shown in Fig. 132, in that a single battery rather than two batteries is used, the current being supplied to one of the lines through the impedance coils _1_ and _2_, and to the other line through the impedance coils _3_ and _4_; condensers _5_ and _6_ serve conductively to isolate the two lines. At the subscriber's station the line circuit pa.s.ses through the secondary of an induction coil and the transmitter.
The receiver is kept entirely in a local circuit so that there is no tendency for direct current to flow through it, but it is receptive to voice currents through the electromagnetic induction between the primary and the secondary of the induction coil.
[Ill.u.s.tration: Fig. 134. Stromberg-Carlson System]
[Ill.u.s.tration: Fig. 135. North Electric Company System]
_North._ Another arrangement of central-office battery feed is employed by the North Electric Company, and is shown in Fig. 135. In this two batteries are used which supply current respectively to the two connected lines, condensers being employed to conductively isolate the lines. This differs from the Kellogg arrangement shown in Fig. 132 in that the two coils _1_ and _2_ are wound on the same core, while the coils _3_ and _4_ are wound together upon another core. In this case, in order that the inductive action of one of the coils may not neutralize that of the other coil on the same core, the two coils are wound in such relative direction that their magnetizing influence will always be c.u.mulative rather than differential.
The central-office arrangements discussed in Figs. 130 to 135, inclusive, are those which are in princ.i.p.al use in commercial practice in common-battery exchanges.
_Current Supply over Limbs of Line in Parallel._ As indicating further interesting possibilities in the method of supplying current from a common source to a number of substations, several other systems will be briefly referred to as being of interest, although these have not gone into wide commercial use. The system shown in Fig. 136 is one proposed by Dean in the early days of common-battery working, and this arrangement was put into actual service and gave satisfactory results, but was afterwards supplanted by the Bell equipment operating under the system shown in Fig. 130, which became standardized by that company. In this the current from the common battery at the central office is not fed over the two line wires in series, but in multiple, using a ground return from the subscriber's station to the central office. Across the metallic circuit formed by two connected lines there is bridged, at the central office, an impedance coil _1_, and between the center point of this impedance coil and the ground is connected the common battery. At the subscriber's station is placed an impedance coil _2_, also bridged across the two limbs of the line, and between the center point of this impedance coil and the ground is connected the transmitter, which is shunted by the primary winding of an induction coil. Connected between the two limbs of the line at the substation there is also the receiver and the secondary of an induction coil in series.
[Ill.u.s.tration: Fig. 136. Current Supply over Parallel Limbs of Line]
The action of this circuit at first seems a little complex, but if taken step by step may readily be understood. The transmitter supply circuit may be traced from the central-office battery through the two halves of the impedance coil _1_ in multiple; thence over the two limbs of the line in multiple to Station A, for instance; thence in multiple through the two halves of impedance coil _2_, to the center point of that coil; thence through the two paths offered respectively by the primary of the induction coil and by the transmitter; then to ground and back to the other pole of the central-office battery. By this circuit the transmitter at the substation is supplied with current.