_Dry Paper. _The demands of telephonic practice are, however, such as to require condensers of very cheap construction with large capacity in a small s.p.a.ce. For this purpose thin bond paper, saturated with paraffin, has been found to be the best dielectric. The conductors in condensers are almost always of tinfoil, this being an ideal material on account of its cheapness and its thinness. Before telephony made such urgent demands for a cheap compact condenser, the customary way of making them was to lay up alternate sheets of dielectric material, either of oiled paper or mica and tinfoil, the sheets of tinfoil being cut somewhat smaller than the sheets of dielectric material in order that the proper insulation might be secured at the edges. After a sufficient number of such plates were built up the alternate sheets of tinfoil were connected together to form one composite plate of the condenser, while the other sheets were similarly connected together to form the other plate. Obviously, in this way a very large area of plates could be secured with a minimum degree of separation.
[Ill.u.s.tration: Fig. 122. Rolled Condenser]
There has been developed for use in telephony, however, and its use has since extended into other arts requiring condensers, what is called the _rolled condenser_. This is formed by rolling together in a flat roll four sheets of thin bond paper, _1_, _2_, _3_, and _4_, and two somewhat narrower strips of tinfoil, _5_ and _6_, Fig. 121. The strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases. Thin sheet metal terminals _7_ and _8_ are rolled into the condenser as it is being wound, and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished. After it is rolled, the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture. It is then squeezed in a press and allowed to cool while under pressure. In this way the surplus paraffin is expelled and the plates are brought very close together. It then appears as in Fig. 122. The condenser is now sealed in a metallic case, usually rectangular in form, and presents the appearance shown in Fig. 123.
[Ill.u.s.tration: Fig. 123. Rolled Condenser]
A later method of condenser making which has not yet been thoroughly proven in practice, but which bids fair to produce good results, varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material. This conducting material is of metallic nature and in reality forms a part of the paper. To form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before.
Sizes. The condensers ordinarily used in telephone practice range in capacity from about 1/4 microfarad to 2 microfarads. When larger capacities than 2 microfarads are desired, they may be obtained by connecting several of the smaller size condensers in multiple. Table IX gives the capacity, shape, and dimensions of a variety of condensers selected from those regularly on the market.
TABLE IX
Condenser Data
+------------+---------------+---------------------------------+DIMENSIONS IN INCHESCAPACITYSHAPE----------+----------+-----------+HeightWidthThickness+------------+---------------+----------+----------+-----------+2 m. f.Rectangular9-1/64-3/411/161 m. f."9-1/64-3/411/161 m. f."4-3/42-3/3213/161/2 m. f."2-3/41-1/43/41 m. f."4-13/162-1/3225/321/2 m. f."4-3/42-3/3213/163/10 m. f."4-3/42-3/3213/161 m. f."2-3/43l+------------+---------------+----------+----------+-----------+
Conventional Symbols. The conventional symbols usually employed to represent condensers in telephone diagrams are shown in Fig. 124.
These all convey the idea of the adjacent conducting plates separated by insulating material.
[Ill.u.s.tration: Fig. 124. Condenser Symbols]
Functions. Obviously, when placed in a circuit a condenser offers a complete barrier to the flow of direct current, since no conducting path exists between its terminals, the dielectric offering a very high insulation resistance. If, however, the condenser is connected across the terminals of a source of alternating current, this current flows first in one direction and then in the other, the electromotive force in the circuit increasing from zero to a maximum in one direction, and then decreasing back to zero and to a maximum in the other direction, and so on. With a condenser connected so as to be subjected to such alternating electromotive forces, as the electromotive force begins to rise the electromotive force at the condenser terminals will also rise and a current will, therefore, flow into the condenser. When the electromotive force reaches its maximum, the condenser will have received its full charge for that potential, and the current flow into it will cease. When the electromotive force begins to fall, the condenser can no longer retain its charge and a current will, therefore, flow out of it. Apparently, therefore, there is a flow of current through the condenser the same as if it were a conductor.
Means for a.s.sorting Currents. In conclusion, it is obvious that the telephone engineer has within his reach in the various coils--whether non-inductive or inductive, or whether having one or several windings--and in the condenser, a variety of tools by which he may achieve a great many useful ends in his circuit work. Obviously, the condenser affords a means for transmitting voice currents or fluctuating currents, and for excluding steady currents. Likewise the impedance coil affords a means for readily transmitting steady currents but practically excluding voice currents or fluctuating currents. By the use of these very simple devices it is possible to sift out the voice currents from a circuit containing both steady and fluctuating currents, or it is possible in the same manner to sift out the steady currents and to leave the voice currents alone to traverse the circuit.
Great use is made in the design of telephone circuits of the fact that the electromagnets, which accomplish the useful mechanical results in causing the movement of parts, possess the quality of impedance. Thus, the magnets which operate various signaling relays at the central office are often used also as impedance coils in portions of the circuit through which it is desired to have only steady currents pa.s.s.
If, on the other hand, it is necessary to place a relay magnet, having considerable impedance, directly in a talking circuit, the bad effects of this on the voice currents may be eliminated by shunting this coil with a condenser, or with a comparatively high non-inductive resistance. The voice currents will flow around the high impedance of the relay coil through the condenser or resistance, while the steady currents, which are the ones which must be depended upon to operate the relay, are still forced in whole or in part to pa.s.s through the relay coil where they belong.
In a similar way the induction coil affords a means for keeping two circuits completely isolated so far as the direct flow of current between them is concerned, and yet of readily transmitting, by electromagnetic induction, currents from one of these circuits to the other. Here is a means of isolation so far as direct current is concerned, with complete communication for alternating current.
CHAPTER XIII
CURRENT SUPPLY TO TRANSMITTERS
The methods by which current is supplied to the transmitter of a telephone for energizing it, may be cla.s.sified under two divisions: first, those where the battery or other source of current is located at the station with the transmitter which it supplies; and second, those where the battery or other source of current is located at a distant point from the transmitter, the battery in such cases serving as a common source of current for the supply of transmitters at a number of stations.
The advantages of putting the transmitter and the battery which supplies it with current in a local circuit with the primary of an induction coil, and placing the secondary of the induction coil in the line, have already been pointed out but may be briefly summarized as follows: When the transmitter is placed directly in the _line circuit_ and the line is of considerable length, the current which pa.s.ses through the transmitter is necessarily rather small unless a battery of high potential is used; and, furthermore, the total change in resistance which the transmitter is capable of producing is but a small proportion of the total resistance of the line, and, therefore, the current changes produced by the transmitter are relatively small.
On the other hand, when the transmitter is placed in a _local circuit_ with the battery, this circuit may be of small resistance and the current relatively large, even though supplied by a low-voltage battery; so that the transmitter is capable of producing relatively large changes in a relatively large current.
To draw a comparison between these two general cla.s.ses of transmitter current supply, a number of cases will be considered in connection with the following figures, in each of which two stations connected by a telephone line are shown. Brief reference to the local battery method of supplying current will be made in order to make this chapter contain, as far as possible, all of the commonly used methods of current supply to transmitters.
[Ill.u.s.tration: A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT]
Local Battery. In Fig. 125 two stations are shown connected by a grounded line wire. The transmitter of each station is included in a low-resistance primary circuit including a battery and the primary winding of an induction coil, the relation between the primary circuits and the line circuits being established by the inductive action between the primary and the secondary windings of induction coils, the secondary in each case being in the line circuits with the receivers.
[Ill.u.s.tration: Fig. 125. Local-Battery Stations with Grounded Circuit]
Fig. 126 shows exactly the same arrangement but with a metallic circuit rather than a grounded circuit. The student should become accustomed to the replacing of one of the line wires of a metallic circuit by the earth, and to the method, employed in Figs. 125 and 126, of indicating a grounded circuit as distinguished from a metallic circuit.
[Ill.u.s.tration: Fig. 126. Local-Battery Stations with Metallic Circuit]
In Fig. 127 is shown a slight modification of the circuit shown in Fig. 126, which consists of connecting one end of the primary winding to one end of the secondary winding of the induction coil, thus linking together the primary circuit and the line circuit, a portion of each of these circuits being common to a short piece of the local wiring. There is no difference whatever in the action of the circuits shown in Figs. 126 and 127, the latter being shown merely for the purpose of bringing out this fact. It is very common, particularly in local-battery circuits, to connect one end of the primary and the secondary windings, as by doing so it is often possible to save a contact point in the hook switch and also to simplify the wiring.
[Ill.u.s.tration: Fig. 127. Local-Battery Stations with Metallic Circuit]
The advantages to be gained by employing a local battery at each subscriber"s station a.s.sociated with the transmitter in the primary circuit of an induction coil are attended by certain disadvantages from a commercial standpoint. The primary battery is not an economical way to generate electric energy. In all its commercial forms it involves the consumption of zinc and zinc is an expensive fuel. The actual amount of current in watts required by a telephone is small, however, and this disadvantage due to the inexpensive method of generating current would not in itself be of great importance. A more serious objection to the use of local batteries at subscribers"
stations appears when the subject is considered from the standpoint of maintenance. Batteries, whether of the so-called "dry" or "wet" type, gradually deteriorate, even when not used, and in cases where the telephone is used many times a day the deterioration is comparatively rapid. This makes necessary the occasional renewals of the batteries with the attendant expense for new batteries or new material, and of labor and transportation in visiting the station. The labor item becomes more serious when the stations are scattered in a spa.r.s.ely settled community, in which case the visiting of the stations, even for the performance of a task that would require but a few minutes" time, may consume some hours on the part of the employes in getting there and back.
Common Battery. _Advantages._ It would be more economical if all of the current for the subscribers" transmitters could be supplied from a single comparatively efficient generating source instead of from a mult.i.tude of inefficient small sources scattered throughout the community served by the exchange. The advantage of such centralization lies not only in more economic generating means, but also in having the common source of current located at one place, where it may be cared for with a minimum amount of expense. Such considerations have resulted in the so-called "common-battery system," wherein the current for all the subscribers" transmitters is furnished from a source located at the central office.
Where such a method of supplying current is practiced, the result has also been, in nearly all cases, the doing away with the subscriber"s magneto generators, relying on the central-office source of current to furnish the energy for enabling the subscriber to signal the operator.
Such systems, therefore, concentrate all of the sources of energy at the central office and for that reason they are frequently referred to as central-energy systems.
NOTE. In this chapter the central-energy or common-battery system will be considered only in so far as the supply of current for energizing the subscribers" transmitters is concerned, the discussion of the action of signaling being reserved for subsequent chapters.
_Series Battery._ If but a single pair of lines had to be considered, the arrangement shown in Fig. 128 might be employed. In this the battery is located at the central office and placed in series with the two grounded lines leading from the central office to the two subscribers" stations. The voltage of this battery is made sufficient to furnish the required current over the resistance of the entire line circuit with its included instruments. Obviously, changes in resistance in the transmitter at Station A will affect the flow of current in the entire line and the fluctuations resulting from the vibration of the transmitter diaphragm will, therefore, reproduce these sounds in the receiver at Station B, as well as in that at Station A.
[Ill.u.s.tration: Fig. 128. Battery in Series with Two Lines]
An exactly similar arrangement applied to a metallic circuit is shown in Fig. 129. In thus placing the battery in series in the circuit between the two stations, as shown in Figs. 128 and 129, it is obvious that the transmitter at each station is compelled to vary the resistance of the entire circuit comprising the two lines in series, in order to affect the receiver at distant stations. This is in effect making the transmitter circuit twice as long as is necessary, as will be shown in the subsequent systems considered. Furthermore, the placing of the battery in series in the circuit of the two combined lines does not lend itself readily to the supply of current from a common source to more than a single pair of lines.
[Ill.u.s.tration: Fig. 129. Battery in Series with Two Lines]
_Series Substation Circuit._ The arrangement at the substations--consisting in placing the transmitter and the receiver in series in the line circuit, as shown in Figs. 128 and 129--is the simplest possible one, and has been used to a considerable extent, but it has been subject to the serious objection, where receivers having permanent magnets were used, of making it necessary to so connect the receiver in the line circuit that the steady current from the battery would not set up a magnetization in the cores of the receiver in such a direction as to neutralize or oppose the magnetization of the permanent magnets. As long as the current flowed through the receiver coils in such a direction as to supplement the magnetization of the permanent magnets, no harm was usually done, but when the current flowed through the receiver coils in such a way as to neutralize or oppose the magnetizing force of the permanent magnets, the action of the receiver was greatly interfered with. As a result, it was necessary to always connect the receivers in the line circuit in a certain way, and this operation was called _poling_.
In order to obviate the necessity for poling and also to bring about other desirable features, it has been, until recently, almost universal practice to so arrange the receiver that it would be in the circuit of the voice currents pa.s.sing over the line, but would not be traversed by direct currents, this condition being brought about by various arrangements of condensers, impedance coils, or induction coils, as will be shown later. During the year 1909, however, the adoption by several concerns of the so-called "direct-current"
receiver has made it necessary for the direct current to flow through the receiver coils in order to give the proper magnetization to the receiver cores, and this has brought about a return to the very simple form of substation circuit, which includes the receiver and the transmitter directly in the circuit of the line. This ill.u.s.trates well an occurrence that is frequently observed by those who have opportunity to watch closely the development of an art. At one time the conditions will be such as to call for complicated arrangements, and for years the aim of inventors will be to perfect these arrangements; then, after they are perfected, adopted, and standardized, a new idea, or a slight alteration in the practice in some other respect, will demand a return to the first principles and wipe out the necessity for the things that have been so arduously striven for.
[Ill.u.s.tration: Fig. 130. Bridging Battery with Repeating Coil]
_Bridging Battery with Repeating Coil._ As pointed out, the placing of the battery in series in the line circuit in the central office is not desirable, and, so far as we are aware, has never been extensively used. The universal practice, therefore, is to place it in a bridge path across the line circuit, and a number of arrangements employing this basic idea are in wide use. In Fig. 130 is shown the standard arrangement of the Western Electric Company, employed by practically all the Bell operating companies. In this the battery at the central office is connected in the middle of the two sides of a repeating coil so that the current from the battery is fed out to the two connected lines in multiple.
Referring to the middle portion of this figure showing the central-office apparatus, _1_ and _2_ may be considered as the two halves of one side of a repeating coil divided so that the battery may be cut into their circuit. Likewise, _3_ and _4_ may be considered as the two halves of the other side of the repeating coil similarly divided for the same purpose. The windings of this repeating coil are ordinarily alike; that is, _1_ and _2_ combined have the same resistance, number of turns, and impedance as _3_ and _4_ combined.
The two sides of this coil are alternately used as primary and secondary, _1_ and _2_ forming the primary when Station A is talking, and _3_ and _4_, the secondary; and _vice versa_ when Station B is talking.
As will be seen, the current flowing from the positive pole of the battery will divide and flow through the windings _2_ and _4_; thence over the upper limb of each line, through the transmitter at each station, and back over the lower limbs of the line, through the windings _1_ and _3_, where the two paths reunite and pa.s.s to the negative pole of the battery. It is evident that when neither transmitter is being used the current flowing through both lines will be a steady current and that, therefore, neither line will have an inductive effect on the other. When, however, the transmitter at Station A is used the variations in the resistance caused by it will cause undulations in the current. These undulations, pa.s.sing through the windings _1_ and _2_ of the repeating coil, will cause, by electromagnetic induction, alternating currents to flow in the windings _3_ and _4_, and these alternating currents will be superimposed on the steady currents flowing in that line and will affect the receiver at Station B, as will be pointed out. The reverse conditions exist when Station B is talking.
_Bell Substation Arrangement._ The substation circuits at the stations in Fig. 130 are ill.u.s.trative of one of the commonly employed methods of preventing the steady current from the battery from flowing through the receiver coil. This particular arrangement is that employed by the common-battery instruments of the various Bell companies. Considering the action at Station B, it is evident that the steady current will pa.s.s through the transmitter and through the secondary winding of the induction coil, and that as long as this current is steady no current will flow through the telephone receiver. The receiver, transmitter, and primary winding of the induction coil are, however, included in a local circuit with the condenser. The presence of the condenser precludes the possibility of direct current flowing in this path.
Considering Station A as a receiving station, it is evident that the voice currents coming to the station over the line will pa.s.s through the secondary winding and will induce alternating currents in the primary winding which will circulate through the local circuit containing the receiver and the condenser, and thus actuate the receiver. The considerations are not so simple when the station is being treated as a transmitting station. Under this condition the steady current pa.s.ses through the transmitter in an obvious manner. It is clear that if the local circuit containing the receiver did not exist, the circuit would be operative as a transmitting circuit because the transmitter would produce fluctuations in the steady current flowing in the line and thus be able to affect the distant station. The transmitter, therefore, has a direct action on the currents flowing in the line by the variation in resistance which it produces in the line circuit. There is, however, a subsidiary action in this circuit. Obviously, there is a drop of potential across the transmitter terminals due to the flow of steady current. This means that the upper terminal of the condenser will be charged to the same potential as the upper terminal of the transmitter, while the lower terminal of the condenser will be of the same potential as the lower terminal of the transmitter. When, now, the transmitter varies its resistance, a variation in the potential across its terminals will occur; and as a result, a variation in potential across the terminals of the condenser will occur, and this means that alternating currents will flow through the primary winding of the induction coil. The transmitter, therefore, by its action, causes alternating currents to flow through the primary of this induction coil and it causes, by direct action on the circuit of the line, fluctuations in the steady current flowing in the line. The alternating currents flowing in the primary of the coil induce currents in the secondary of the coil which supplement and augment the fluctuations produced by the direct action of the transmitter. This circuit may be looked at, therefore, in the light of combining the direct action which the transmitter produces in the current in the line with the action which the transmitter produces in the local circuit containing the primary of the induction coil, this action being repeated in the line circuit through the secondary of the induction coil.
The receiver in this circuit is placed in the local circuit, and is thus not traversed by the steady currents flowing in the line. There is thus no necessity for poling it. This circuit is very efficient, but is subject to the objection of producing a heavy side tone in the receiver of the transmitting station. By "side tone" is meant the noises which are produced in the receiver at a station by virtue of the action of the transmitter at that station. Side tone is objectionable for several reasons: first, it is sometimes annoying to the subscriber; second, and of more importance, the subscriber who is talking, hearing a very loud noise in his own receiver, unconsciously a.s.sumes that he is talking too loud and, therefore, lowers his voice, sometimes to such an extent that it will not properly reach the distant station.
[Ill.u.s.tration: Fig. 131. Bridging Battery with Impedance Coils]