Cyclopedia of Telephony and Telegraphy Volume I Part 18

_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.

Variations in the resistance of the transmitter when in action, cause complementary variations in the supply current flowing through the primary of the induction coil. These variations induce similar alternating currents in the secondary of this coil, which is in series in the line circuit. The currents, so induced in this secondary, flow in series through one side of the line to the distant station; thence through the secondary and the receiver at that station to the other side of the line and back through that side of the line to the receiver. These currents are not permitted to pa.s.s through the bridged paths across the metallic circuit that are offered by the impedance coils _1_ and _2_, because they are voice currents and are, therefore, debarred from these paths by virtue of the impedance.

[Ill.u.s.tration: Fig. 137. Current Supply over Parallel Limbs of Line]

An objection to this form of current supply and to other similar forms, wherein the transmitter current is fed over the two sides of the line in multiple with a ground return, is that the ground-return circuit formed by the two sides of the line in multiple is subject to inductive disturbances from other lines in the same way as an ordinary grounded line is subject to inductive disturbance. The current-supply circuit is thus subject to external disturbances and such disturbances find their way into the metallic circuit and, therefore, through the instruments by means of the electromagnetic induction between the primary and the secondary coils at the substations.

Another interesting method of current supply from a central-office battery is shown in Fig. 137. This, like the circuit just considered, feeds the energy to the subscriber's station over the two sides of the line in multiple with a ground return. In this case, however, a local circuit is provided at the substation, in which is placed a storage battery _1_ and the primary _2_ of an induction coil, together with the transmitter. The idea in this is that the current supply from the central office will pa.s.s through the storage battery and charge it.

Upon the use of the transmitter, this storage battery acts to supply current to the local circuit containing the transmitter and the primary coil _2_ in exactly the same manner as in a local battery system. The fluctuating current so produced by the action of the transmitter in this local circuit acts on the secondary winding _3_ of the induction coil, and produces therein alternating currents which pa.s.s to the central office and are in turn repeated to the distant station.

_Supply Many Lines from Common Source._ We come now to the consideration of the arrangement by which a single battery may be made to supply current at the central office to a large number of pairs of connected lines simultaneously. Up to this point in this discussion it has been shown only how each battery served a single pair of connected lines and no others.

Repeating Coil:--In Fig. 138 is shown how a single battery supplies current simultaneously to four different pairs of lines, the lines of each pair being connected for conversation. It is seen that the pairs of lines shown in this figure are arranged in each case in accordance with the system shown in Fig. 130. Let us inquire why it is that, although all of these four pairs of lines are connected with a common source of energy and are, therefore, all conductively joined, the stations will be able to communicate in pairs without interference between the pairs. In other words, why is it that voice currents originating at Station A will pa.s.s only to the receiver at Station B and not to the receivers at Station C or Station H, for instance? The reason is that separate supply conductors lead from the points such as _1_ and _2_ at the junctions of the repeating-coil windings on each pair of circuits to the battery terminals, and the resistance and impedance of the battery itself and of the common leads to it are so small that although the feeble voice currents originating in the pair of lines connecting Station A and Station B pa.s.s through the battery, they are not able to alter the potential of the battery in any appreciable degree. As a result, therefore, the supply wires leading from the common-battery terminals to the points _7_ and _8_, for instance, cannot be subjected to any variations in potential by virtue of currents flowing through the battery from the points _1_ and _2_ of the lines joining Station A and Station B.

[Ill.u.s.tration: MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA, PA.]

[Ill.u.s.tration: Fig. 138. Common Source for Many Lines]

[Ill.u.s.tration: Fig. 139. Common Source for Many Lines]

r.e.t.a.r.dation Coil--Single Battery:--In Fig. 139 is shown in similar manner the current supply from a single battery to four different pairs of lines, the battery being a.s.sociated with the lines by the combined impedance coil and condenser method, which was specifically dealt with in connection with Fig. 133. The reasons why there will be no interference between the conversations carried on in the various pairs of connected lines in this case are the same as those just considered in connection with the system shown in Fig. 138. The impedance coils in this case serve to keep the telephone currents confined to their respective pairs of lines in which they originate, and this same consideration applies to the system of Fig. 138, for each of the separate repeating-coil windings of Fig. 138 is in itself an impedance coil with respect to such currents as might leak away from one pair of lines on to another.

r.e.t.a.r.dation Coil--Double Battery:--The arrangement of feeding a number of pairs of lines according to the Kellogg two-battery system is indicated in Fig. 140, which needs no further explanation in view of the description of the preceding figures. It is interesting to note in this case that the left-hand battery serves only the left-hand lines and the right-hand battery only the right-hand lines. As this is worked out in practice, the left-hand battery is always connected to those lines which originate a call and the right-hand battery always to those lines that are called for. The energy supplied to a calling line is always, therefore, from a different source than that which supplies a called line.

[Ill.u.s.tration: Fig. 140. Two Sources for Many Lines]

[Ill.u.s.tration: Fig. 141. Current Supply from Distant Point]

_Current Supply from Distant Point._ Sometimes it is convenient to supply current to a group of lines centering at a certain point from a source of current located at a distant point. This is often the case in the so-called private branch exchange, where a given business house or other inst.i.tution is provided with its own switchboard for interconnecting the lines leading to the various telephones of that concern or inst.i.tution among themselves, and also for connecting them with lines leading to the city exchange. It is not always easy or convenient to maintain at such private switchboards a separate battery for supplying the current needed by the local exchange.

In such cases the arrangement shown in Fig. 141 is sometimes employed.

This shows two pairs of lines connected by the impedance-coil system with common terminals _1_ and _2_, between which ordinarily the common battery would be connected. Instead of putting a battery between these terminals, however, at the local exchange, a condenser of large capacity is connected between them and from these terminals circuit wires _3_ and _4_ are led to a battery of suitable voltage at a distant central office. The condenser in this case is used to afford a short-circuit path for the voice currents that leak from one side of one pair of lines to the other, through the impedance coils bridged across the line. In this way the effect of the necessarily high resistance in the common leads _3_ and _4_, leading to the storage battery, is overcome and the tendency to cross-talk between the various pairs of connected lines is eliminated. Frequently, instead of employing this arrangement, a storage battery of small capacity will be connected between the terminals _1_ and _2_, instead of the condenser, and these will be charged over the wires _3_ and _4_ from a source of current at a distant point.

A consideration of the various methods of supplying current from a common source to a number of lines will show that it is essential that the resistance of the battery itself be very low. It is also necessary that the resistance and the impedance of the common leads from the battery to the point of distribution to the various pairs of lines be very low, in order that the voice currents which flow through them, by virtue of the conversations going on in the different pairs of lines, shall not produce any appreciable alteration in the difference of potential between the battery terminals.

CHAPTER XIV

THE TELEPHONE SET

We have considered what may be called the elemental parts of a complete telephone; that is, the receiver, transmitter, hook switch, battery, generator, call bell, condenser, and the various kinds of coils which go to make up the apparatus by which one is enabled to transmit and receive speech and signals. We will now consider the grouping of these various elements into a complete working organization known as a telephone.

Before considering the various types it is well to state that the term telephone is often rather loosely used. We sometimes hear the receiver proper called a telephone or a hand telephone. Since this was the original speaking telephone, there is some reason for so calling the receiver. The modern custom more often applies the term telephone to the complete organization of talking and signaling apparatus, together with the a.s.sociated wiring and cabinet or standard on which it is mounted. The name telephone set is perhaps to be preferred to the word telephone, since it tends to avoid misunderstanding as to exactly what is meant. Frequently, also, the telephone or telephone set is referred to as a subscriber's station equipment, indicating the equipment that is to be found at a subscriber's station. This, as applying to a telephone alone, is not proper, since the subscriber's station equipment includes more than a telephone. It includes the local wiring within the premises of the subscriber and also the lightning arrester and other protective devices, if such exist.

To avoid confusion, therefore, the collection of talking and signaling apparatus with its wiring and containing cabinet or standard will be referred to in this work as a telephone or telephone set. The receiver will, as a rule, be designated as such, rather than as a telephone.

The term subscriber's station equipment will refer to the complete equipment at a subscriber's station, and will include the telephone set, the interior wiring, and the protective devices, together with any other apparatus that may be a.s.sociated with the telephone line and be located within the subscriber's premises.

Cla.s.sification of Sets. Telephones may be cla.s.sified under two general headings, magneto telephones and common-battery telephones, according to the character of the systems in which they are adapted to work.