Station A the upper conductor Fig
Station A the upper conductor, Fig. 170, is connected to binding post
_1_ and the lower conductor to binding post _2_, while at Station B
the upper conductor is connected to binding post _2_ and the lower
conductor to binding post _1_. The permanent wiring of this telephone
set is the same as that frequently used for a set connected to a line
having only one station, the proper ringing circuit being made by the
method of connecting up the binding posts. For example, if this
telephone set were to be used on a single station line, the binding
posts _1_ and _2_ would be connected to the two conductors of the line
as before, while binding post _3_ would be connected to post _1_
instead of being grounded.
[Illustration: Fig. 175. Circuit of Two-Party Station]
_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
set used with the system illustrated in Fig. 172 is shown in detail in
Fig. 176. The wiring of this set is arranged for local battery or
magneto working, as this method of selective ringing is more frequently
employed with magneto systems, on account of the objectionable features
which arise when applied to common-battery systems. In this figure the
line conductors are connected to binding posts _1_ and _2_, and a
ground connection is made to binding post _3_. In order that all sets
may be wired alike and yet permit the instrument to be connected for
any one of the various stations, the bell is not permanently wired to
any portion of the circuit but has flexible connections which will
allow of the set being properly connected for any desired station. The
terminals of the bell are connected to binding posts _9_ and _10_, to
which are connected flexible conductors terminating in terminals _7_
and _8_. These terminals may be connected to the binding posts _4_,
_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
D station, as required. For example, in connecting the set for Station
A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
_5_. For connecting the set for Station B terminal _7_ is connected to
binding post _5_ and _8_ to _6_. For connecting the set for Station C
terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
connecting the set for Station D terminal _7_ is connected to binding
post _4_ and _8_ to _6_.
[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]
[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]
The detailed wiring of the telephone set employed in connection with
the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
this set is arranged for a common-battery system, inasmuch as this
arrangement of signaling circuit is more especially adapted for
common-battery working. However, this arrangement is frequently
adapted to magneto systems as even with magneto systems a permanent
ground connection at a subscribers station is objectionable inasmuch
as it increases the difficulty of determining the existence or
location of an accidental ground on one of the line conductors. The
wiring of this set is also arranged so that one standard type of
wiring may be employed and yet allow any telephone set to be connected
as an A, B, C, or D station.
Harmonic Method. _Principles._ To best understand the principle of
operation of the harmonic party-line signaling systems, it is to be
remembered that a flexible reed, mounted rigidly at one end and having
its other end free to vibrate, will, like a violin string, have a
certain natural period of vibration; that is, if it be started in
vibration, as by snapping it with the fingers, it will take up a
certain rate of vibration which will continue at a uniform rate until
the vibration ceases altogether. Such a reed will be most easily
thrown into vibration by a series of impulses having a frequency
corresponding exactly to the natural rate of vibration of the reed
itself; it may be thrown into vibration by very slight impulses if
they occur at exactly the proper times.
It is familiar to all that a person pushing another in a swing may
cause a considerable amplitude of vibration with the exertion of but a
small amount of force, if he will so time his pushes as to conform
exactly to the natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.