Several important points must be borne in mind
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.