._ In other cases, where several telephones are placed on a
single-line circuit, the bells are arranged in multiple across the
line. For this reason their magnets are wound with a very great number
of turns and consequently to a high resistance. In order to further
increase the impedance, the cores are made long and heavy. Since the
generators on these lines must be capable of giving out a sufficient
volume of current to divide up between all of the bells in multiple,
it follows that these generators must have a large current output, and
at the same time a sufficient voltage to ring the bells at the
farthest end of the line. Such instruments are commonly called
bridging instruments, on account of the method of connecting their
bells across the circuit of the line.

[Illustration: Fig. 148. Circuit of Bridging Magneto Set]

The fundamental characteristic of the bridging telephone is that it
contains three possible bridge paths across the line wires. The first
of these bridge paths is through the talking apparatus, the second
through the generator, and the third through the ringer. This is shown
in simplified form in Fig. 148. The talking apparatus is associated
with the two upper contacts of the hook switch in the usual manner and
needs no further description. The generator is the second separate
bridge path, normally open, but adapted to be closed when the
generator is operated, this automatic closure being performed by the
movement of the crank shaft. The third bridge contains the polarized
bell, and this, as a rule, is permanently closed. Sometimes, however,
the arrangement is such that the bell path is normally closed through
the switch which is operated by the generator crank shaft, and this
path is automatically broken when the generator is operated, at which
time, also, the generator path is automatically closed. This
arrangement brings about the result that the generator never can ring
its own bell, because its switch always operates to cut out the bell
at its own station just before the generator itself is cut into the
circuit.

In Fig. 149 is shown the complete circuit of a bridging telephone.
The circuit given in this figure is for a local-battery wall set
similar in type to that shown in Figs. 142 and 143. A simplified
diagrammatic arrangement is shown in the lower left-hand corner of
this figure, and from a consideration of this it will be seen that the
bell circuit across the line is normally completed t