Chapter 16

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 associated 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 associated with the telephone line and be located within the subscriber's premises.

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

_Magneto Telephone._ The term magneto telephone, as it was originally employed in telephony, referred to the type of instrument now known as a receiver, particularly when this was used also as a transmitter. As the use of this instrument as a transmitter has practically ceased, the term magneto telephone has lost its significance as applying to the receiver, and, since many telephones are equipped with magneto generators for calling purposes, the term magneto telephone has, by common consent, come to be used to designate any telephone including, as a part of its equipment, a magneto generator. Magneto telephones usually, also, include local batteries for furnishing the transmitter with current, and this has led to these telephones being frequently called local battery telephones. However, a local battery telephone is not necessarily a magneto telephone and _vice versâ_, since sometimes magneto telephones have no local batteries and sometimes local battery telephones have no magnetos. Nearly all of the telephones which are equipped with magneto generators are, however, also equipped with local batteries for talking purposes, and, therefore, the terms magneto telephone and local battery telephone usually refer to the same thing.

_Common-Battery Telephone._ Common-battery telephones, on the other hand, are those which have no local battery and no magneto generator, all the current for both talking and signaling being furnished from a common source of current at the central office.

_Wall and Desk Telephones._ Again we may classify telephones or telephone sets in accordance with the manner in which their various parts are associated with each other for use, regardless of what parts are contained in the set. We may refer to all sets adapted to be mounted on a wall or partition as _wall telephones_, and to all in which the receiver, transmitter, and hook are provided with a standard of their own to enable them to rest on any flat surface, such as a desk or table, as _desk telephones_. These latter are also referred to as portable telephones and as portable desk telephones.

In general, magneto or local battery telephones differ from common-battery telephones in their component parts, the difference residing principally in the fact that the magneto telephone always has a magneto generator and usually a local battery, while the common-battery telephone has no local source of current whatever. On the other hand, the differences between wall telephones and desk telephones are principally structural, and obviously either of these types of telephones may be for common-battery or magneto work. The same component parts go to make up a desk telephone as a wall telephone, provided the two instruments are adapted for the same class of service, but the difference between the two lies in the structural features by which these same parts are associated with each other and protected from exposure.

Magneto-Telephone Sets. _Wall._ In Fig. 142 is shown a familiar type of wall set. The containing box includes within it all of the working parts of the apparatus except that which is necessarily left outside in order to be within the reach of the user. Fig. 143 shows the same set with the door open. This gives a good idea of the ordinary arrangement of the apparatus within. It is seen that the polarized bell or ringer has its working parts mounted on the inside of the door or cover of the box, the tapper projecting through so as to play between the gongs on the outside. Likewise the transmitter arm, which supports the transmitter and allows its adjustment up and down to accommodate itself to the height of the user, is mounted on the front of the door, and the conductors leading to it may be seen fastened to the rear of the door in Fig. 143.

In some wall sets the wires leading to the bell and transmitter are connected to the wiring of the rest of the set through the hinges of the door, thus allowing the door to be opened and closed repeatedly without breaking off the wires. In order to always insure positive electrical contact between the stationary and movable parts of the hinge a small wire is wound around the hinge pin, one end being soldered to the stationary part and the other end to the movable part of the hinge. In other forms of wall set the wires to the bell and the transmitter lead directly from the stationary portion of the cabinet to the back of the door, the wires being left long enough to have sufficient flexibility to allow the door to be opened and closed without injuring the wires.

At the upper portion of the box there is mounted the hook switch, this being, in this case, of the short lever type. The lever of the hook projects through the side of the box so as to make the hook available as a support for the receiver. Immediately at the right of the hook switch is mounted the induction coil, and immediately below this the generator, its crank handle projecting through the right-hand side of the box so as to be available for use there. The generator is usually mounted on a transverse shelf across the middle of the cabinet, this shelf serving to form a compartment below it in which the dry battery of two or three cells is placed.

The wall telephone-set cabinets have assumed a multitude of forms. When wet cells rather than dry cells were ordinarily employed, as was the case up to about the year 1895, the magneto generator, polarized bell, and hook switch were usually mounted in a rectangular box placed at the top of a long backboard. Immediately below this on the backboard was mounted the transmitter arm, and sometimes the base of this included the induction coil. Below this was the battery box, this being a large affair usually adapted to accommodate two and sometimes three ordinary LeClanché cells side by side.

The dry cell has almost completely replaced the wet cell in this country, and as a result, the general type of wall set as shown in Figs. 142 and 143, has gradually replaced the old wet-cell type, which was more cumbrous and unsightly. It is usual on wall sets to provide some sort of a shelf, as indicated in Fig. 142, for the convenience of the user in making notes and memoranda.

_Desk._ In the magneto desk-telephone sets, the so-called desk stand, containing the transmitter, the receiver, and the hook switch, with the standard upon which they are mounted, is shown in Fig. 144. This desk stand evidently does not comprise the complete equipment for a magneto desk-telephone set, since the generator, polarized bell, and battery are lacking. The generator and bell are usually mounted together in a box, either on the under side of the desk of the user or on the wall within easy reach of his chair. Connections are made between the apparatus in the desk stand proper and the battery, generator, and bell by means of flexible conducting cords, these carrying a plurality of conductors, as required by the particular circuit of the telephone in question. Such a complete magneto desk-telephone set is shown in Fig. 145, this being one of the types manufactured by the Stromberg-Carlson Manufacturing Company.

A great variety of arrangements of the various parts of magneto desk-telephone apparatus is employed in practice. Sometimes, as shown in Fig. 145, the magneto bell box is equipped with binding posts for terminating all of the conductors in the cord, the line wires also running to some of these binding posts.

In the magneto-telephone set illustrated the box is made large enough to accommodate only the generator and call bell, and the batteries are mounted elsewhere, as in a drawer of the desk, while in other cases there is no other equipment but that shown in the cut, the batteries being mounted within the magneto bell box itself. In still other cases, the polarized bell is contained in one box, the generator in another, the batteries in the drawer of the desk, the induction coil being mounted either in the base of the desk stand, in the bell box, or in the generator box. In such cases all of the circuits of the various scattered parts are wired to a terminal strip, located at some convenient point, this strip containing terminals for all the wires leading from the various parts and for the line wires themselves. By combining the various wires on the terminals of this terminal strip, the complete circuits of the telephone are built up. In still other cases the induction coil is mounted on the terminal strip and separate wires or sets of wires are run to the polarized bell and generator, to the desk stand itself, and to the batteries. These various arrangements are subject largely to the desire or personal ideas of the manufacturer or user. All of them work on the same principle so far as the operation of the talking and signaling circuits is concerned.

Circuits of Magneto-Telephone Sets. Magneto telephones, whether of the wall or desk type, may be divided into two general classes, series and bridging, according to whether the magnet of the bell is included in series or bridge relation with the telephone line when the hook is down.

_Series._ In the so-called series telephone line, where several telephones are placed in series in a single line circuit, the employment of the series type of telephone results in all of the telephone bells being in series in the line circuit. This means that the voice currents originating in the telephones that are in use at a given time must pass in series through the magnets of the bells of the stations that are not in use. In order that these magnets, through which the voice currents must pass, may interfere to as small a degree as possible with the voice currents, it is common to employ low-resistance magnets in series telephones, these magnets being wound with comparatively few turns and on rather short cores so that the impedance will be as small as possible. Likewise, since the generators are required to ring all of the bells in series, they need not have a large current output, but must have sufficient voltage to ring through all of the bells in series and through the resistance of the line. For this reason the generators are usually of the three-bar type and sometimes have only two bars.

In Fig. 146 are shown, in simplified form, the circuits of an ordinary series telephone. The receiver in this is shown as being removed from the hook and thus the talking apparatus is brought into play. The line wires _1_ and _2_ connect respectively to the binding posts _3_ and _4_ which form the terminals of the instrument. When the hook is up, the circuit between the binding posts _3_ and _4_ includes the receiver and the secondary winding of the induction coil, together with one of the upper contacts _5_ of the switch hook and the hook lever itself. This completes the circuit for receiving speech. The hook switch is provided with another upper contact _6_, between which and the contact _5_ is connected the local circuit containing the transmitter, the battery, and the primary of the induction coil in series. The primary and the secondary windings are connected together at one end and connected with the switch contact _5_, as shown. It is thus seen that when the hook is up the circuit through the receiver is automatically closed and also the local circuit containing the primary, the battery, and the transmitter. Thus, all the conditions for transmitting and receiving speech are fulfilled.

[Fig. 146. Circuit of Series Magneto Set]

When the hook is down, however, the receiving and transmitting circuits are broken, but another circuit is completed by the engagement of the hook-switch lever with the lower hook contact _7_. Between this contact and one side of the line is connected the polarized ringer and the generator. With the hook down, therefore, the circuit may be traced from the line wire _1_ to binding post _3_, thence through the generator shunt to the call bell, and thence through the lower switching contact _7_ to the binding post _4_ and line wire _2_. The generator shunt, as already described in Chapter VIII, normally keeps the generator shunted out of circuit. When, however, the generator is operated the shunt is broken, which allows the armature of the generator to come into the circuit in series with the winding of the polarized bell. The normal shunting of the generator armature from the circuit of the line is advantageous in several ways. In the first place, the impedance of the generator winding is normally cut out of the circuit so that in the case of a line with several stations the talking or voice currents do not have to flow through the generator armatures at the stations which are not in use. Again, the normal shunting of the generator tends to save the generator armature from injury by lightning.

The more complete circuits of a series magneto telephone are shown in Fig. 147. In this the line binding posts are shown as _1_ and _2_. At the bottom of the telephone cabinet are four other binding posts marked _3_, _4_, _5_, and _6_. Of these _3_ and _4_ serve for the receiver terminals and _5_ and _6_ for the transmitter and battery terminals. The circuits of this diagram will be found to be essentially the same as those of Fig. 146, except that they are shown in greater detail. This particular type of circuit is one commonly employed where the generator, ringer, hook switch, and induction coil are all mounted in a so-called magneto bell box at the top of the instrument, and where the transmitter is mounted on an arm just below this box, and the battery in a separate compartment below the transmitter. The only wiring that has to be done between the bell box and the other parts of the instrument in assembling the complete telephone is to connect the receiver to the binding posts _3_ and _4_ and to connect the battery and transmitter circuit to the binding posts _5_ and _6_.

_Bridging._ In other cases, where several telephones are placed on a single-line circuit, the bells are arranged in multiple across the line. For this reason their magnets are wound with a very great number of turns and consequently to a high resistance. In order to further increase the impedance, the cores are made long and heavy. Since the generators on these lines must be capable of giving out a sufficient volume of current to divide up between all of the bells in multiple, it follows that these generators must have a large current output, and at the same time a sufficient voltage to ring the bells at the farthest end of the line. Such instruments are commonly called bridging instruments, on account of the method of connecting their bells across the circuit of the line.

The fundamental characteristic of the bridging telephone is that it contains three possible bridge paths across the line wires. The first of these bridge paths is through the talking apparatus, the second through the generator, and the third through the ringer. This is shown in simplified form in Fig. 148. The talking apparatus is associated with the two upper contacts of the hook switch in the usual manner and needs no further description. The generator is the second separate bridge path, normally open, but adapted to be closed when the generator is operated, this automatic closure being performed by the movement of the crank shaft. The third bridge contains the polarized bell, and this, as a rule, is permanently closed. Sometimes, however, the arrangement is such that the bell path is normally closed through the switch which is operated by the generator crank shaft, and this path is automatically broken when the generator is operated, at which time, also, the generator path is automatically closed. This arrangement brings about the result that the generator never can ring its own bell, because its switch always operates to cut out the bell at its own station just before the generator itself is cut into the circuit.

In Fig. 149 is shown the complete circuit of a bridging telephone. The circuit given in this figure is for a local-battery wall set similar in type to that shown in Figs. 142 and 143. A simplified diagrammatic arrangement is shown in the lower left-hand corner of this figure, and from a consideration of this it will be seen that the bell circuit across the line is normally completed through the two right-hand normally closed contacts of the switch on the generator. When, however, the generator is operated these two contacts are made to disengage each other while the long spring of the generator switch engages the left-hand spring and thus brings the generator itself into the circuit.

Of the three binding posts, _1_, _2_, and _3_, at the top of Fig. 149, _1_ and _2_ are for connecting with the line wires, while _8_ is for a ground connection, acting in conjunction with the lightning arrester mounted at the top of the telephone and indicated at _4_ in Fig. 149. This has no function in talking or ringing, and will be referred to more fully in Chapter XIX. Suffice it to say at this point that these arresters usually consist of two conducting bodies, one connected permanently to each of the line binding posts, and a third conducting body connected to the ground binding post. These three conducting bodies are in close proximity but carefully insulated from each other; the idea being that when the line wires are struck by lightning or subjected otherwise to a dangerous potential, the charge on the line will jump across the space between the conducting bodies and pass harmlessly to ground.

NOTE. The student should practice making simplified diagrams from actual wiring diagrams. The difference between the two is that one is laid out for ease in understanding it, while the other is laid out to show the actual course of the wires as installed.

If the large detailed circuit of Fig. 149 be compared with the small theoretical circuit in the same figure, the various conducting paths will be found to be the same. Such a simplified circuit does more to enable one to grasp the fundamental scheme of a complex circuit than much description, since it shows at a glance the general arrangement. The more detailed circuits are, however, necessary to show the actual paths followed by the wiring.

The circuits of desk stands do not differ from those of wall sets in any material degree, except as may be necessitated by the fact that the various parts of the telephone set are not all mounted in the same cabinet or on the same standard. To provide for the necessary relative movement between the desk stand and the other portions of the set, flexible conductors are run from the desk stand itself to the stationary portions of the equipment, such as the battery and the parts contained in the generator and bell box.

In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto desk-telephone set, illustrated in Fig. 145. This diagram needs no explanation in view of what has already been said. The conductors, leading from the desk-stand group of apparatus to the bell-box group of apparatus, are grouped together in a flexible cord, as shown in Fig. 145, and are connected respectively to the various binding posts or contact points within the desk stand at one end and at the base of the bell box at the other end. These flexible conductors are insulated individually and covered by a common braided covering. They usually are individualized by having a colored thread woven into their insulating braid, so that it is an easy matter to identify the two ends of the same conductor at either end of the flexible cord or cable.

Common-Battery Telephone Sets. Owing to the fact that common-battery telephones contain no sources of current, they are usually somewhat simpler than the magneto type. The component parts of a common-battery telephone, whether of the wall or desk type, are the transmitter, receiver, hook switch, polarized bell, condenser, and sometimes an induction coil. The purpose of the condenser is to prevent direct or steady currents from passing through the windings of the ringer while the ringer is connected across the circuit of the line during the time when the telephone is not in use. The requirements of common-battery signaling demand that the ringer shall be connected with the line so as to be receptive of a call at any time while the telephone is not in use. The requirements also demand that no conducting path shall normally exist between the two sides of the line. These two apparently contradictory requirements are met by placing a condenser in series with the ringer so that the ringer will be in a path that will readily transmit the alternating ringing currents sent out from the central-office generator, while at the same time the condenser will afford a complete bar to the passage of steady currents. Sometimes the condenser is also used as a portion of the talking apparatus, as will be pointed out.

_Wall._ In Figs. 151 and 152 are given two views of a characteristic form of common-battery wall-telephone set, made by the Stromberg-Carlson Manufacturing Company. The common-battery wall set has usually taken this general form. In it the transmitter is mounted on an adjustable arm at the top of the backboard, while the box containing the bell and all working parts of the instrument is placed below the transmitter, the top of the box affording a shelf for writing purposes. In Fig. 151 are shown the hook switch and the receiver; just below these may be seen the magnets of the polarized bell, back of which is shown a rectangular box containing the condenser. Immediately in front of the ringer magnets is the induction coil.

In Fig. 153 are shown the details of the circuit of this instrument. This figure also includes a simplified circuit arrangement from which the principles involved may be more readily understood. It is seen that the primary of the induction coil and the transmitter are included in series across the line. The secondary of the induction coil, in series with the receiver, is connected also across the line in series with a condenser and the transmitter.

_Hotel._ Sometimes, in order to economize space, the shelf of common-battery wall sets is omitted and the entire apparatus mounted in a small rectangular box, the front of which carries the transmitter mounted on the short arm or on no arm at all. Such instruments are commonly termed hotel sets, because of the fact that their use was first confined largely to the rooms in hotels. Later, however, these instruments have become very popular in general use, particularly in residences. Sometimes the boxes or cabinets of these sets are made of wood, but of recent years the tendency has been growing to make them of pressed steel. The steel box is usually finished in black enamel, baked on, the color being sometimes varied to match the color of the surrounding woodwork. In Figs. 154 and 155 are shown two views of a common-battery hotel set manufactured by the Dean Electric Company.

Such sets are extremely neat in appearance and have the advantage of taking up little room on the wall and the commercial advantage of being light and compact for shipping purposes. A possible disadvantage of this type of instrument is the somewhat crowded condition which necessarily follows from the placing of all the parts in so confined a space. This interferes somewhat with the accessibility of the various parts, but great ingenuity has been manifested in making the parts readily get-at-able in case of necessity for repairs or alterations.

_Desk_. The common-battery desk telephone presents a somewhat simpler problem than the magneto desk telephone for the reason that the generator and local battery, the two most bulky parts of a magneto telephone, do not have to be provided for. Some companies, in manufacturing desk stands for common-battery purposes, mount the condenser and the induction coil or impedance coil, or whatever device is used in connection with the talking circuit, in the base of the desk stand itself, and mount the polarized ringer and the condenser used for ringing purposes in a separate bell box adapted to be mounted on the wall or some portion of the desk. Other companies mount only the transmitter, receiver, and hook switch on the desk stand proper and put the condenser or induction coil, or other device associated with the talking circuit, in the bell box. There is little to choose between the two general practices. The number of conducting strands in the flexible cord is somewhat dependent on the arrangement of the circuit employed.

The Kellogg Switchboard and Supply Company is one which places all the parts, except the polarized ringer and the associated condenser, in the desk stand itself. In Fig. 156 is shown a bottom view of the desk stand with the bottom plate removed. In the upper portion of the circle of the base is shown a small condenser which is placed in the talking circuit in series with the receiver. In the right-hand portion of the circle of the base is shown a small impedance coil, which is placed in series with the transmitter but in shunt relation with the condenser and the receiver.

In Figs. 157 and 158 are shown two views of the type of bell box employed by the Kellogg Company in connection with the common-battery desk sets, this box being of pressed-steel construction and having a removable lid, as shown in Fig. 158, by which the working parts of the ringer are made readily accessible, as are also the terminals for the cord leading from the desk stand and for the wires of the line circuit. The condenser that is placed in series with the ringer is also mounted in this same box. By employing two condensers, one in the bell box large enough to transmit ringing currents and the other in the base of the desk stand large enough only to transmit voice currents, a duplication of condensers is involved, but it has the corresponding advantages of requiring only two strands to the flexible cord leading from the bell box to the desk stand proper.

A form of desk-telephone set that is used largely abroad, but that has found very little use in this country, is shown in Fig. 159. In this the transmitter and the receiver are permanently attached together, the receiver being of the watch-case variety and so positioned relatively to the transmitter that when the receiver is held at the ear, the mouthpiece of the transmitter will be just in front of the lips of the user. In order to maintain the transmitter in a vertical position during use, this necessitates the use of a curved mouthpiece as shown. This transmitter and receiver so combined is commonly called, in this country, the _microtelephone set_, although there seems to be no logical reason for this name. The combined transmitter and receiver, instead of being supported on an ordinary form of hook switch, are supported on a forked bracket as shown, this bracket serving to operate the switch springs which are held in one position when the bracket is subjected to the weight of the microtelephone, and in the alternate position when relieved therefrom. This particular microtelephone set is the product of the L.M. Ericsson Telephone Manufacturing Company, of Buffalo, New York. The circuits of such sets do not differ materially from those of the ordinary desk telephone set.

Circuits of Common-Battery Telephone Sets. The complete circuits of the Kellogg desk-stand arrangement are shown in Fig. 160, the desk-stand parts being shown at the left and the bell-box parts at the right. As is seen, but two conductors extend from the former to the latter. A simplified theoretical sketch is also shown in the upper right-hand corner of this figure.

The details of the common-battery telephone circuits of the Dean Electric Company are shown in Fig. 161. This involves the use of the balanced Wheatstone bridge. The only other thing about this circuit that needs description, in view of what has previously been said about it, is that the polarized bell is placed in series with a condenser so that the two sides of the circuit may be insulated from each other while the telephone is not in use, and yet permit the passage of ringing current through the bell.

The use of the so-called direct-current receiver has brought about a great simplification in the common-battery telephone circuits of several of the manufacturing companies. By this use the transmitter and the receiver are placed in series across the line, this path being normally opened by the hook-switch contacts. The polarized bell and condenser are placed in another bridge path across the line, this path not being affected by the hook-switch contacts. All that there is to such a complete common-battery telephone set, therefore, is a receiver, transmitter, hook switch, bell, condenser, and cabinet, or other support.

The extreme simplicity of the circuits of such a set is illustrated in Fig. 162, which shows how the Monarch Telephone Manufacturing Company connect up the various parts of their telephone set, using the direct-current receiver already described in connection with Fig. 54.