Volume I Part 12 (2/2)
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 bus.h.i.+ngs 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 establis.h.i.+ng 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 s.h.i.+pment 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.
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