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, principally
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 assume 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 associated with the
line at the central office. This use of the hook switch in the case of
the common-battery telephone is a good illustration 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 space 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 assure 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 accumulated, 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 substitute 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-class apparatus. The various cheap alloys intended as
substitutes 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.
[Illustration: 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 space 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.
[Illustration: 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.
[Illustration: 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 space available for the switch contacts in the desk
telephone.
[Illustration: 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 brass tube
_1_, attached to the base by a screw-threaded joint, as shown. The
switch lever _2_ is pivoted at _3_ in a brass plug _4_, closing the
upper end of the tube forming the standard. This brass 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 brass 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.
[Illustration: 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.
[Illustration: 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.
Conventional Symbols. The hook switch plays a very important part
in the operation of telephone circuits; for this reason readily
understood conventional symbols, by which they may be conveniently
represented in drawings of circuits, are desirable. In Fig. 88 are
shown several symbols such as would apply to almost any circuit,
regardless of the actual mechanical details of the particular hook
switch which happened to be employed. Thus diagram _A_ in Fig. 88
shows a hook switch having a single make contact and this diagram
might be used to refer to the hook switch of the Dean Electric Company
shown in Fig. 85, in which only a single contact is made when the
receiver is removed, and none is made when it is on the hook.
Similarly, diagram _B_ might be used to represent the hook switch of
the Kellogg Company, shown in Fig. 83, the arrangement being for two
make and two break contacts. Likewise diagram _C_ might be used to
represent the hook switch of the Western Electric Company, shown in
Fig. 84, which, as before stated, has two make contacts only. Diagram
_D_ shows another modification in which contacts made by the hook
switch, when the receiver is removed, control two separate circuits.
Assuming that the solid black portion represents insulation, it is
obvious that the contacts are divided into two groups, one insulated
from the other.
[Illustration: Fig. 88. Hook Switch Symbols]
[Illustration: COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND
CHIPPING IN MANHOLES]