Therefore, to secure high impedance we need many turns or low reluctance, or both. Often, owing to requirements for direct-current carrying capacity and limitations of s.p.a.ce, a very large number of turns is not permissible, in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance, usually a completely closed circuit.
_Kind of Iron. _An important factor in the design of impedance coils is the grade of iron used in the magnetic circuit. Obviously, it should be of the highest permeability and, furthermore, there should be ample cross-section of core to prevent even an approach to saturation. The iron should, if possible, be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects.
_Types._ Open-Circuit:--Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and ill.u.s.trating particularly the method of bringing out the terminals of the winding.
[Ill.u.s.tration: Fig. 101. Section of Open-Circuit Impedance Coil]
[Ill.u.s.tration: Fig. 102. Open-Circuit Impedance Coil]
[Ill.u.s.tration: Fig. 103. Closed-Circuit Impedance Coil]
Closed-Circuit:--A type of r.e.t.a.r.dation coil which is largely used in systems of simultaneous telegraphy and telephony, known as _composite systems_, is shown in Fig. 103. In the construction of this coil the core is made of a bundle of fine iron wires first bent into U-shape, and then after the coils are in place, the free ends of the core are brought together to form a closed magnetic circuit. The coils have a large number of turns of rather coa.r.s.e wire. The conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity, and fortunately the added requirement, that it shall be placed in a very small s.p.a.ce, does not exist.
Toroidal:--Another type of r.e.t.a.r.dation coil, called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire, is shown in diagram in Fig. 104.
The two windings of this coil may be connected in series to form in effect a single winding, or it may be used as a "split-winding" coil, the two windings being in series but having some other element, such as a battery, connected between them in the circuit. Evidently such a coil, however connected, is well adapted for high impedance, on account of the low reluctance of its core.
[Ill.u.s.tration: Fig. 104. Symbol of Toroidal Impedance Coil]
This coil is usually mounted on a base-board, the coil being enclosed in a protecting iron case, as shown in Fig. 105. The terminal wires of both windings of each coil are brought out to terminal punchings on one end of the base-board to facilitate the making of the necessary circuit connections.
[Ill.u.s.tration: Fig. 105. Toroidal Impedance Coil]
The usual diagrammatic symbol for an impedance coil is shown in Fig.
106. This is the same as for an ordinary bar magnet, except that the parallel lines through the core may be taken as indicating that the core is laminated, thus conveying the idea of high impedance. The symbol of Fig. 104 is a good one for the toroidal type of impedance coil.
[Ill.u.s.tration: Fig. 106. Symbol of Impedance Coil]
Induction Coil. An induction coil consists of two or more windings of wire interlinked by a common magnetic circuit. In an induction coil having two windings, any change in the strength of the current flowing in one of the windings, called the _primary_, will cause corresponding changes in the magnetic flux threading the magnetic circuit, and, therefore, changes in flux through the other winding, called the _secondary_. This, by the laws of electromagnetic induction, will produce corresponding electromotive forces in the secondary winding and, therefore, corresponding currents in that winding if its circuit be closed.
_Current and Voltage Ratios._ In a well-designed induction coil the energy in the secondary, _i.e._, the induced current, is for all practical purposes equal to that of the primary current, yet the values of the voltage and the amperage of the induced current may vary widely from the values of the voltage and the amperage of the primary current. With simple periodic currents, such as the commercial alternating lighting currents, the ratio between the voltage in the primary and that in the secondary will be equal to the ratio of the number of turns in the primary to the number of turns in the secondary. Since the energy in the two circuits will be practically the same, it follows _that the ratio between the current in the primary and that in the secondary will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary_. In telephony, where the currents are not simple periodic currents, and where the variations in current strength take place at different rates, such a law as that just stated does not hold for all cases; but it may be stated in general that _the induced currents will be of higher voltage and smaller current strength than those of the primary in all coils where the secondary winding has a greater number of turns than the primary_, and _vice versa_.
_Functions._ The function of the induction coil in telephony is, therefore, mainly one of transformation, that is, either of stepping up the voltage of a current, or in other cases stepping it down. The induction coil, however, does serve another purpose in cases where no change in voltage and current strength is desired, that is, it serves as a means for electrically separating two circuits so far as any conductive relation exists, and yet of allowing the free transmission by induction from one of these circuits to the other. This is a function that in telephony is scarcely of less importance than the purely transforming function.
_Design._ Induction coils, as employed in telephony, may be divided into two general types: first, those having an open magnetic circuit; and, second, those having a closed magnetic circuit. In the design of either type it is important that the core should be thoroughly laminated, and this is done usually by forming it of a bundle of soft Swedish or Norway iron wire about .02 of an inch in diameter. The diameter and the length of the coil, and the relation between the number of turns in the primary and in the secondary, and the mechanical construction of the coil, are all matters which are subject to very wide variation in practice. While the proper relationship of these factors is of great importance, yet they may not be readily determined except by actual experiment with various coils, owing to the extreme complexity of the action which takes place in them and to the difficulty of obtaining fundamental data as to the existing facts.
It may be stated, therefore, that the design of induction coils is nearly always carried out by "cut-and-try" methods, bringing to bear, of course, such scientific and practical knowledge as the experimenter may possess.
[Ill.u.s.tration: Fig. 107. Induction Coil]
[Ill.u.s.tration: Fig. 108. Section of Induction Coil]
_Use and Advantage._ The use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters. Such induction coils are nearly always of the open magnetic circuit type, consisting of a long, straight core comprised of a bundle of small annealed iron wires, on which is wound a primary of comparatively coa.r.s.e wire and having a small number of turns, and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns. A view of such a coil mounted on a base is shown in Fig. 107, and a sectional view of a similar coil is shown in Fig. 108. The method of bringing out the winding terminals is clearly indicated in this figure, the terminal wires _2_ and _4_ being those of the primary winding and _1_ and _3_ those of the secondary winding. It is customary to bring out these wires and attach them by solder to suitable terminal clips. In the case of the coil shown in Fig. 108 these clips are mounted on the wooden heads of the coil, while in the design shown in Fig. 107 they are mounted on the base, as is clearly indicated.
Repeating Coil. The so-called repeating coil used in telephony is really nothing but an induction coil. It is used in a variety of ways and usually has for its purpose the inductive a.s.sociation of two circuits that are conductively separated. Usually the repeating coil has a one to one ratio of turns, that is, there are the same number of turns in the primary as in the secondary. However, this is not always the case, since sometimes they are made to have an unequal number of turns, in which case they are called _step-up _or _step-down_ repeating coils, according to whether the primary has a smaller or a greater number of turns than the secondary. Repeating coils are almost universally of the closed magnetic circuit type.
_Ringing and Talking Considerations._ Since repeating coils often serve to connect two telephones, it follows that it is sometimes necessary to ring through them as well as talk through them. By this is meant that it is necessary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents. Ringing currents ordinarily have a frequency ranging from about 16 to 75 cycles per second, while voice currents have frequencies ranging from a few hundred up to perhaps ten thousand per second. Ordinarily, therefore, the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and _vice versa_.
If the comparatively heavy ringing currents alone were to be considered, the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit. On the other hand, for carrying voice currents alone it is usually made with a small amount of iron and with small windings, in order to prevent waste of energy in the core, and to give a high degree of responsiveness with the least amount of distortion of wave form, so that the voice currents will retain as far as possible their original characteristics. When, therefore, a coil is required to carry both ringing and talking currents, a compromise must be effected.
_Types._ The form of repeating coil largely used for both ringing and talking through is shown in Fig. 109. This coil comprises a soft iron core made up of a bundle of wires about .02 inch in diameter, the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core. The windings of this particular coil are four in number, and contain about 2,400 turns each, and have a resistance of about 60 ohms. In this coil, when connected for local battery work, the windings are connected in pairs in series, thus forming effectively two windings having about 120 ohms resistance each. The whole coil is enclosed in a protecting case of iron. The terminals are brought out to suitable clips on the wooden base, as shown. An external perspective view of this coil is shown in Fig. 110.
By bringing out each terminal of each winding, eight in all, as shown in this figure, great lat.i.tude of connection is provided for, since the windings may be connected in circuit in any desirable way, either by connecting them together in pairs to form virtually a primary and a secondary, or, as is frequently the case, to split the primary and the secondary, connecting a battery between each pair of windings.
[Ill.u.s.tration: Fig. 109. Repeating Coil]
[Ill.u.s.tration: Fig. 110. Repeating Coil]
Fig. 111 ill.u.s.trates in section a commercial type of coil designed for talking through only. This coil is provided with four windings of 1,357 turns each, and when used for local battery work the coils are connected in pairs in series, thus giving a resistance of about 190 ohms in each half of the repeating coil. The core of this coil consists of a bundle of soft iron wires, and the sh.e.l.l which forms the return path for the magnetic lines is of very soft sheet iron. This sh.e.l.l is drawn into cup shape and its open end is closed, after the coil is inserted, by the insertion of a soft iron head, as indicated.
As in the case of the coil shown in Figs. 109 and 110, eight terminals are brought out on this coil, thus providing the necessary flexibility of connection.
[Ill.u.s.tration: Fig. 111. Repeating Coil]
[Ill.u.s.tration: Fig. 112. Diagram of Toroidal Repeating Coil]
[Ill.u.s.tration: Fig. 113. Toroidal Repeating Coil]
Still another type of repeating coil is ill.u.s.trated in diagram in Fig.
112, and in view in Fig. 113. This coil, like the impedance coil shown in Fig. 104, comprises a core made up of a bundle of soft iron wires wound into the form of a ring. It is usually provided with two primary windings placed opposite each other upon the core, and with two secondary windings, one over each primary. In practice these two primary windings are connected in one circuit and the two secondaries in another. This is the standard repeating coil now used by the Bell companies in their common-battery cord circuits.
[Ill.u.s.tration: THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY, MISSOURI]
[Ill.u.s.tration: Fig. 114. Symbol of Induction Coil]
Conventional Symbols. The ordinary symbol for the induction coil used in local battery work is shown in Fig. 114. This consists merely of a pair of parallel zig-zag lines. The primary winding is usually indicated by a heavy line having a fewer number of zig-zags, and the secondary by a finer line having a greater number of zig-zags. In this way the fact that the primary is of large wire and of comparatively few turns is indicated. This diagrammatic symbol may be modified to suit almost any conditions, and where a tertiary as well as a secondary winding is provided it may be shown by merely adding another zig-zag line.
[Ill.u.s.tration: Fig. 115. Repeating-Coil Symbols]
The repeating coil is indicated symbolically in the two diagrams of Fig. 115. Where there is no necessity for indicating the internal connections of the coil, the symbol shown in the left of this figure is usually employed. Where, however, the coil consists of four windings rather than two and the method of connecting them is to be indicated, the symbol at the right hand is employed. In Fig. 116 another way of indicating a four-winding repeating coil or induction coil is shown. Sometimes such windings may be combined by connection to form merely a primary and a secondary winding, and in other cases the four windings all act separately, in which case one may be considered the primary and the others, respectively, the secondary, tertiary, and quaternary.
[Ill.u.s.tration: Fig. 116. Symbol of Four-Winding Repeating Coil]
Where the toroidal type of repeating coil is employed, the diagram of Fig. 112, already referred to, is a good symbolic representation.
CHAPTER XI
NON-INDUCTIVE RESISTANCE DEVICES
It is often desired to introduce simple ohmic resistance into telephone circuits, in order to limit the current flow, or to create specific differences of potential at given points in the circuit.
Temperature Coefficient. The design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed.
The resistance of conductors is subject to change by changes in temperature. While nearly all metals show an increase, carbon shows a decrease in its resistance when heated.
The temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree.
TABLE V
Temperature Coefficients