04.22.08
This defect may in some measure be reduced by
This defect may in some measure be reduced by making the ringers of
low impedance. This is the general practice with series telephones,
the ringers ordinarily having short cores and a comparatively small
number of turns, the resistance being as a rule about 80 ohms.
Bridging Systems. Very much better than the series plan of
party-line connections, is the arrangement by which the instruments
are placed in bridges across the line, such lines being commonly known
as bridged or bridging lines. This was first strongly advocated and
put into wide practical use by J.J. Carty, now the Chief Engineer of
the American Telephone and Telegraph Company.
A simple illustration of a bridging telephone line is shown in Fig.
166, where the three telephones shown are each connected in a bridge
path from the line wire to ground, a type known as a “grounded
bridging line.” Its use is very common in rural districts.
A better arrangement is shown in Fig. 167, which represents a
metallic-circuit bridging line, three telephone instruments being
shown in parallel or bridge paths across the two line wires.
The actual circuit arrangements of a bridging party line are better
shown in Fig. 168. There are three stations and it will be seen that
at each station there are three possible bridges, or bridge paths,
across the two limbs of the line. The first of these bridges is
controlled by the hook switch and is normally open. When the hook is
raised, however, this path is closed through the receiver and
secondary of the induction coil, the primary circuit being also closed
so as to include the battery and transmitter. This constitutes an
ordinary local-battery talking set.
[Illustration: Fig. 166. Grounded Bridging Line]
[Illustration: Fig. 167. Metallic Bridging Line]
[Illustration: Fig. 168. Metallic Bridging Line]
A second bridge at each station is led through the ringer or
call-bell, and this, in most bridging telephones, is permanently
closed, the continuity of this path between the two limbs of the line
not being affected either by the hook switch or by the automatic
switch in connection with the generator.
A third bridge path at each station is led through the generator.
This, as indicated, is normally open, but the automatic cut-in switch
of the generator serves, when the generator is operated, to close its
path across the line, so that it may send its currents to the line and
ring the bells of all the stations.
When any generator is operated, its current divides and passes over
the line wires and through all of the ringers in multiple. It is seen,
therefore, that the requirements for a bridging generator are that it
shall be capable of generating a large current, sufficient when
divided up amongst all the bells to ring each of them; and that it
shall be capable of producing a sufficient voltage to send the
required current not only to the near-by stations, but to the stations
at the distant end of the line.
It might seem at first that the bridging system avoided one difficulty
only to encounter another. It clearly avoids the difficulty of the
series system in that the voice currents, in order to reach distant
stations, do not have to pass through all of the bells of the idle
stations in series. There is, however, presented at each station a
leakage path through the bell bridged across the line, through which
it would appear the voice currents might leak uselessly from one side
of the line to the other and not pass on in sufficient volume to the
distant station. This difficulty is, however, more apparent than real.
It is found that, by making the ringers of high impedance, the leakage
of voice currents through them from one side of the line to the other
is practically negligible.
It is obvious that in a heavily loaded bridged line, the bell at the
home station, that is at the station from which the call is being sent,
will take slightly more than its share of the current, and it is also
obvious that the ringing of the home bell performs no useful function.
The plan is frequently adopted, therefore, of having the operation of
the generator serve to cut its own bell out of the circuit. The
arrangement by which this is done is clearly shown in Fig. 169. The
circuit of the bell is normally complete across the line, while the
circuit of the generator is normally open. When, however, the generator
crank is turned these conditions are reversed, the bell circuit being
broken and the generator circuit closed, so as to allow its current all
to pass the line. This feature of having the local bell remain silent
upon the operation of its own generator is also of advantage because
other parties at the same station are not disturbed by the ringing of
the bell when a call is being made by that station.