_Western Electric Ringer._ A typical form of polarized bell is shown in Fig. 79, this being the standard bell or ringer of the Western Electric Company. The two electromagnets are mounted side by side, as shown, by attaching their cores to a yoke piece _1_ of soft iron. This yoke piece also carries the standards _2_ upon which the gongs are mounted. The method of mounting is such that the standards may be adjusted slightly so as to bring the gongs closer _to_ or farther _from_, the tapper.
The soft iron yoke piece _1_ also carries two bra.s.s posts _3_ which, in turn, carry another yoke _4_ of bra.s.s. In this yoke _4_ is pivoted, by means of trunnion screws, the armature _5_, this extending on each side of the pivot so that its ends lie opposite the free poles of the electromagnets. From the center of the armature projects the tapper rod carrying the ball or striker which plays between the two gongs.
In order that the armature and cores may be normally polarized, a permanent magnet _6_ is secured to the center of the yoke piece _1_.
This bends around back of the electromagnets and comes into close proximity to the armature _5_. By this means one end of each of the electromagnet cores is given one polarity--say north--while the armature is given the other polarity--say south. The two coils of the electromagnet are connected together in series in such a way that current in a given direction will act to produce a north pole in one of the free poles and a south pole in the other. If it be a.s.sumed that the permanent magnet maintains the armature normally of south polarity and that the current through the coils is of such direction as to make the left-hand core north and the right-hand core south, then it is evident that the left-hand end of the armature will be attracted and the right-hand end repelled. This will throw the tapper rod to the right and sound the right-hand bell. A reversal in current will obviously produce the opposite effect and cause the tapper to strike the left-hand bell.
An important feature in polarized bells is the adjustment between the armature and the pole pieces. This is secured in the Western Electric bell by means of the nuts _7_, by which the yoke _4_ is secured to the standards _3_. By moving these nuts up or down on the standards the armature may be brought closer _to_ or farther _from_ the poles, and the device affords ready means for clamping the parts into any position to which they may have been adjusted.
[Ill.u.s.tration: Fig. 79. Polarized Bell]
_Kellogg Ringer._ Another typical ringer is that of the Kellogg Switchboard and Supply Company, shown in Fig. 80. This differs from that of the Western Electric Company mainly in the details by which the armature adjustment is obtained. The armature supporting yoke _1_ is attached directly to the cores of the magnets, no supporting side rods being employed. Instead of providing means whereby the armature may be adjusted toward or from the poles, the reverse practice is employed, that is, of making the poles themselves extensible. This is done by means of the iron screws _2_ which form extensions of the cores and which may be made to approach or recede from the armature by turning them in such direction as to screw them in or out of the core ends.
[Ill.u.s.tration: Fig. 80. Polarized Bell]
[Ill.u.s.tration: Fig. 81. Biased Bell]
_Biased Bell._ The pulsating-current generator has already been discussed and its principle of operation pointed out in connection with Fig. 77. The companion piece to this generator is the so-called biased ringer. This is really nothing but a common alternating-current polarized ringer with a light spring so arranged as to hold the armature normally in one of its extreme positions so that the tapper will rest against one of the gongs. Such a ringer is shown in Fig. 81 and needs no further explanation. It is obvious that if a current flows in the coils of such a ringer in a direction tending to move the tapper toward the left, then no sound will result because the tapper is already moved as far as it can be in that direction. If, however, currents in the opposite direction are caused to flow through the windings, then the electromagnetic attraction on the armature will overcome the pull of the spring and the tapper will move over and strike the right-hand gong. A cessation of the current will allow the spring to exert itself and throw the tapper back into engagement with the left-hand gong. A series of such pulsations in the proper direction will, therefore, cause the tapper to play between the two gongs and ring the bell as usual. A series of currents in a wrong direction will, however, produce no effect.
Conventional Symbols. In Fig. 82 are shown six conventional symbols of polarized bells. The three at the top, consisting merely of two circles representing the magnets in plan view, are perhaps to be preferred as they are well standardized, easy to draw, and rather suggestive. The three at the bottom, showing the ringer as a whole in side elevation, are somewhat more specific, but are objectionable in that they take more s.p.a.ce and are not so easily drawn.
[Ill.u.s.tration: Fig. 82. Ringer Symbols]
Symbols _A_ or _B_ may be used for designating any ordinary polarized ringer. Symbols _C_ and _D_ are interchangeably used to indicate a biased ringer. If the bell is designed to operate only on positive impulses, then the plus sign is placed opposite the symbol, while a minus sign so placed indicates that the bell is to be operated only by negative impulses.
Some specific types of ringers are designed to operate only on a given frequency of current. That is, they are so designed as to be responsive to currents having a frequency of sixty cycles per second, for instance, and to be unresponsive to currents of any other frequency. Either symbols _E_ or _F_ may be used to designate such ringers, and if it is desired to indicate the particular frequency of the ringer this is done by adding the proper numeral followed by a short reversed curve sign indicating frequency. Thus 50~ would indicate a frequency of fifty cycles per second.
CHAPTER IX
THE HOOK SWITCH
Purpose. In complete telephone instruments, comprising both talking and signaling apparatus, it is obviously desirable that the two sets of apparatus, for talking and signaling respectively, shall not be connected with the line at the same time. A certain switching device is, therefore, necessary in order that the signaling apparatus alone may be left operatively connected with the line while the instrument is not being used in the transmission of speech, and in order that the signaling apparatus may be cut out when the talking apparatus is brought into play.
In instruments employing batteries for the supply of transmitter current, another switching function is the closing of the battery circuit through the transmitter and the induction coil when the instrument is in use for talking, since to leave the battery circuit closed all the time would be an obvious waste of battery energy.
In the early forms of telephones these switching operations were performed by a manually operated switch, the position of which the user was obliged to change before and after each use of the telephone.
The objection to this was not so much in the manual labor imposed on the user as in the tax on his memory. It was found to be practically a necessity to make this switching function automatic, princ.i.p.ally because of the liability of the user to forget to move the switch to the proper position after using the telephone, resulting not only in the rapid waste of the battery elements but also in the inoperative condition of the signal-receiving bell. The solution of this problem, a vexing one at first, was found in the so-called automatic hook switch or switch hook, by which the circuits of the instrument were made automatically to a.s.sume their proper conditions by the mere act, on the part of the user, of removing the receiver from, or placing it upon, a conveniently arranged hook or fork projecting from the side of the telephone casing.
Automatic Operation. It may be taken as a fundamental principle in the design of any piece of telephone apparatus that is to be generally used by the public, that the necessary acts which a person must perform in order to use the device must, as far as possible, follow as a natural result from some other act which it is perfectly obvious to the user that he must perform. So in the case of the switch hook, the user of a telephone knows that he must take the receiver from its normal support and hold it to his ear; and likewise, when he is through with it, that he must dispose of it by hanging it upon a support obviously provided for that purpose.
In its usual form a forked hook is provided for supporting the receiver in a convenient place. This hook is at the free end of a pivoted lever, which is normally pressed upward by a spring when the receiver is not supported on it. When, however, the receiver is supported on it, the lever is depressed by its weight. The motion of the lever is mechanically imparted to the members of the switch proper, the contacts of which are usually enclosed so as to be out of reach of the user. This switch is so arranged that when the hook is depressed the circuits are held in such condition that the talking apparatus will be cut out, the battery circuit opened, and the signaling apparatus connected with the line. On the other hand, when the hook is in its raised position, the signaling apparatus is cut out, the talking apparatus switched into proper working relation with the line, and the battery circuit closed through the transmitter.
In the so-called common-battery telephones, where no magneto generator or local battery is included in the equipment at the subscriber"s station, the mere raising of the hook serves another important function. It acts, not only to complete the circuit through the substation talking apparatus, but, by virtue of the closure of the line circuit, permits a current to flow over the line from the central-office battery which energizes a signal a.s.sociated with the line at the central office. This use of the hook switch in the case of the common-battery telephone is a good ill.u.s.tration of the principle just laid down as to making all the functions which the subscriber has to perform depend, as far as possible, on acts which his common sense alone tells him he must do. Thus, in the common-battery telephone the subscriber has only to place the receiver at his ear and ask for what he wants. This operation automatically displays a signal at the central office and he does nothing further until the operator inquires for the number that he wants. He has then nothing to do but wait until the called-for party responds, and after the conversation his own personal convenience demands that he shall dispose of the receiver in some way, so he hangs it up on the most convenient object, the hook switch, and thereby not only places the apparatus at his telephone in proper condition to receive another call, but also conveys to the central office the signal for disconnection.
Likewise in the case of telephones operating in connection with automatic exchanges, the hook switch performs a number of functions automatically, of which the subscriber has no conception; and while, in automatic telephones, there are more acts required of the user than in the manual, yet a study of these acts will show that they all follow in a way naturally suggested to the user, so that he need have but the barest fundamental knowledge in order to properly make use of the instrument. In all cases, in properly designed apparatus, the arrangement is such that the failure of the subscriber to do a certain required act will do no damage to the apparatus or to the system, and, therefore, will inconvenience only himself.
Design. The hook switch is in reality a two-position switch, and while at present it is a simple affair, yet its development to its high state of perfection has been slow, and its imperfections in the past have been the cause of much annoyance.
Several important points must be borne in mind in the design of the hook switch. The spring provided to lift the hook must be sufficiently strong to accomplish this purpose and yet must not be strong enough to prevent the weight of the receiver from moving the switch to its other position. The movement of this spring must be somewhat limited in order that it will not break when used a great many times, and also it must be of such material and shape that it will not lose its elasticity with use. The shape and material of the restoring spring are, of course, determined to a considerable extent by the length of the lever arm which acts on the spring, and on the s.p.a.ce which is available for the spring.
The various contacts by which the circuit changes are brought about upon the movement of the hook-switch lever usually take the form of springs of German silver or phosphor-bronze, hard rolled so as to have the necessary resiliency, and these are usually tipped with platinum at the points of contact so as to a.s.sure the necessary character of surface at the points where the electric circuits are made or broken.
A slight sliding movement between each pair of contacts as they are brought together is considered desirable, in that it tends to rub off any dirt that may have acc.u.mulated, yet this sliding movement should not be great, as the surfaces will then cut each other and, therefore, reduce the life of the switch.
Contact Material. On account of the high cost of platinum, much experimental work has been done to find a subst.i.tute metal suitable for the contact points in hook switches and similar uses in the manufacture of telephone apparatus. Platinum is unquestionably the best known material, on account of its non-corrosive and heat-resisting qualities. Hard silver is the next best and is found in some first-cla.s.s apparatus. The various cheap alloys intended as subst.i.tutes for platinum or silver in contact points may be dismissed as worthless, so far as the writers" somewhat extensive investigations have shown.
In the more recent forms of hook switches, the switch lever itself does not form a part of the electrical circuit, but serves merely as the means by which the springs that are concerned in the switching functions are moved into their alternate cooperative relations. One advantage in thus insulating the switch lever from the current-carrying portions of the apparatus and circuits is that, since it necessarily projects from the box or cabinet, it is thus liable to come in contact with the person of the user. By insulating it, all liability of the user receiving shocks by contact with it is eliminated.
Wall Telephone Hooks. _Kellogg._ A typical form of hook switch, as employed in the ordinary wall telephone sets, is shown in Fig. 83, this being the standard hook of the Kellogg Switchboard and Supply Company. In this the lever _1_ is pivoted at the point _3_ in a bracket _5_ that forms the base of all the working parts and the means of securing the entire hook switch to the box or framework of the telephone. This switch lever is normally pressed upward by a spring _2_, mounted on the bracket _5_, and engaging the under side of the hook lever at the point _4_. Attached to the lever arm _1_ is an insulated pin _6_. The contact springs by which the various electrical circuits are made and broken are shown at _7_, _8_, _9_, _10_, and _11_, these being mounted in one group with insulated bushings between them; the entire group is secured by machine screws to a lug projecting horizontally from the bracket _5_. The center spring _9_ is provided with a forked extension which embraces the pin _6_ on the hook lever. It is obvious that an up-and-down motion of the hook lever will move the long spring _9_ in such manner as to cause electrical contact either between it and the two upper springs _7_ and _8_, or between it and the two lower springs _10_ and _11_. The hook is shown in its raised position, which is the position required for talking.
When lowered the two springs _7_ and _8_ are disengaged from the long spring _9_ and from each other, and the three springs _9_, _10_, and _11_ are brought into electrical engagement, thus establishing the necessary signaling conditions.
[Ill.u.s.tration: Fig. 83. Long Lever Hook Switch]
The right-hand ends of the contact springs are shown projecting beyond the insulating supports. This is for the purpose of facilitating making electrical joints between these springs and the various wires which lead from them. These projecting ends are commonly referred to as ears, and are usually provided with holes or notches into which the connecting wire is fastened by soldering.
_Western Electric._ Fig. 84 shows the type of hook switch quite extensively employed by the Western Electric Company in wall telephone sets where the s.p.a.ce is somewhat limited and a compact arrangement is desired. It will readily be seen that the principle on which this hook switch operates is similar to that employed in Fig. 83, although the mechanical arrangement of the parts differs radically. The hook lever _1_ is pivoted at _3_ on a bracket _2_, which serves to support all the other parts of the switch. The contact springs are shown at _4_, _5_, and _6_, and this latter spring _6_ is so designed as to make it serve as an actuating spring for the hook. This is accomplished by having the curved end of this spring press against the lug _7_ of the hook and thus tend to raise the hook when it is relieved of the weight of the receiver. The two shorter springs _8_ and _9_ have no electrical function but merely serve as supports against which the springs _4_ and _5_ may rest, when the receiver is on the hook, these springs _4_ and _5_ being given a light normal tension toward the stop springs _8_ and _9_. It is obvious that in the particular arrangement of the springs in this switch no contacts are closed when the receiver is on the hook.
[Ill.u.s.tration: Fig. 84. Short Lever Hook Switch]
Concerning this latter feature, it will be noted that the particular form of Kellogg hook switch, shown in Fig. 83, makes two contacts and breaks two when it is raised. Similarly the Western Electric Company"s makes two contacts but does not break any when raised. From such considerations it is customary to speak of a hook such as that shown in Fig. 83 as having two make and two break contacts, and such a hook as that shown in Fig. 84 as having two make contacts.
It will be seen from either of these switches that the modification of the spring arrangement, so as to make them include a varying number of make-and-break contacts, is a simple matter, and switches of almost any type are readily modified in this respect.
[Ill.u.s.tration: Fig. 85. Removable Lever Hook Switch]
_Dean_. In Fig. 85 is shown a decidedly unique hook switch for wall telephone sets which forms the standard equipment of the Dean Electric Company. The hook lever _1_ is pivoted at _2_, an auxiliary lever _3_ also being pivoted at the same point. The auxiliary lever _3_ carries at its rear end a slotted lug _4_, which engages the long contact spring _5_, and serves to move it up and down so as to engage and disengage the spring _6_, these two springs being mounted on a base lug extending from the base plate _7_, upon which the entire hook-switch mechanism is mounted. The curved spring _8_, also mounted on this same base, engages the auxiliary lever _3_ at the point _9_ and normally serves to press this up so as to maintain the contact springs _5_ in engagement with contact spring _6_. The switch springs are moved entirely by the auxiliary lever _3_, but in order that this lever _3_ may be moved as required by the hook lever _1_, this lever is provided with a notched lug _10_ on its lower side, which notch is engaged by a forwardly projecting lug _11_ that is integral with the auxiliary lever _3_. The switch lever may be bodily removed from the remaining parts of the hook switch by depressing the lug _11_ with the finger, so that it disengages the notch in lug _10_, and then drawing the hook lever out of engagement with the pivot stud _2_, as shown in the lower portion of the figure. It will be noted that the pivotal end of the hook lever is made with a slot instead of a hole as is the customary practice.
The advantage of being able to remove the hook switch bodily from the other portions arises mainly in connection with the shipment or transportation of instruments. The projecting hooks cause the instruments to take up more room and thus make larger packing boxes necessary than would otherwise be used. Moreover, in handling the telephones in store houses or transporting them to the places where they are to be used, the projecting hook switch is particularly liable to become damaged. It is for convenience under such conditions that the Dean hook switch is made so that the switch lever may be removed bodily and placed, for instance, inside the telephone box for transportation.
Desk-Stand Hooks. The problem of hook-switch design for portable desk telephones, while presenting the same general characteristics, differs in the details of construction on account of the necessarily restricted s.p.a.ce available for the switch contacts in the desk telephone.
[Ill.u.s.tration: WEST OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO Serving the General Western Business and Residence Districts.]
_Western Electric._ In Fig. 86 is shown an excellent example of hook-switch design as applied to the requirements of the ordinary portable desk set. This figure is a cross-sectional view of the base and standard of a familiar type of desk telephone. The base itself is of stamped metal construction, as indicated, and the standard which supports the transmitter and the switch hook for the receiver is composed of a black enameled or nickel-plated bra.s.s tube _1_, attached to the base by a screw-threaded joint, as shown. The switch lever _2_ is pivoted at _3_ in a bra.s.s plug _4_, closing the upper end of the tube forming the standard. This bra.s.s plug supports also the transmitter, which is not shown in this figure. Attached to the plug _4_ by the screw _5_ is a heavy strip _6_, which reaches down through the tube to the base plate of the standard and is held therein by a screw _7_. The plug _4_, carrying with it the switch-hook lever _2_ and the bra.s.s strip _6_, may be lifted bodily out of the standard _1_ by taking out the screw _7_ which holds the strip _6_ in place, as is clearly indicated. On the strip _6_ there is mounted the group of switch springs by which the circuit changes of the instrument are brought about when the hook is raised or lowered. The spring _8_ is longer than the others, and projects upwardly far enough to engage the lug on the switch-hook lever _2_. This spring, which is so bent as to close the contacts at the right when not prevented by the switch lever, also serves as an actuating spring to raise the lever _2_ when the receiver is removed from it. This spring, when the receiver is removed from the hook, engages the two springs at the right, as shown, or when the receiver is placed on the hook, breaks contact with the two right-hand springs and makes contact respectively with the left-hand spring and also with the contact _9_ which forms the transmitter terminal.
[Ill.u.s.tration: Fig. 86. Desk-Stand Hook Switch]
It is seen from an inspection of this switch hook that it has two make and two break contacts. The various contact springs are connected with the several binding posts shown, these forming the connectors for the flexible cord conductors leading into the base and up through the standard of the desk stand. By means of the conductors in this cord the circuits are led to the other parts of the instrument, such as the induction coil, call bell, and generator, if there is one, which, in the case of the Western Electric Company"s desk set, are all mounted separately from the portable desk stand proper.
This hook switch is accessible in an easy manner and yet not subject to the tampering of idle or mischievous persons. By taking out the screw _7_ the entire hook switch may be lifted out of the tube forming the standard, the cords leading to the various binding posts being slid along through the tube. By this means the connections to the hook switch, as well as the contact of the switch itself, are readily inspected or repaired by those whose duty it is to perform such operations.
_Kellogg._ In Fig. 87 is shown a sectional view of the desk-stand hook switch of the Kellogg Switchboard and Supply Company. In this it will be seen that instead of placing the switch-hook springs within the standard or tube, as in the case of the Western Electric Company, they are mounted in the base where they are readily accessible by merely taking off the base plate from the bottom of the stand. The hook lever operates on the long spring of the group of switch springs by means of a toggle joint in an obvious manner. This switch spring itself serves by its own strength to raise the hook lever when released from the weight of the receiver.
[Ill.u.s.tration: Fig. 87. Desk-Stand Hook Switch]
In this switch, the hook lever, and in fact the entire exposed metal portions of the instrument, are insulated from all of the contact springs and, therefore, there is little liability of shocks on the part of the person using the instrument.