down, however, the receiving and transmitting
circuits are broken, but another circuit is completed by the
engagement of the hook-switch lever with the lower hook contact _7_.
Between this contact and one side of the line is connected the
polarized ringer and the generator. With the hook down, therefore, the
circuit may be traced from the line wire _1_ to binding post _3_,
thence through the generator shunt to the call bell, and thence
through the lower switching contact _7_ to the binding post _4_ and
line wire _2_. The generator shunt, as already described in Chapter
VIII, normally keeps the generator shunted out of circuit. When,
however, the generator is operated the shunt is broken, which allows
the armature of the generator to come into the circuit in series with
the winding of the polarized bell. The normal shunting of the
generator armature from the circuit of the line is advantageous in
several ways. In the first place, the impedance of the generator
winding is normally cut out of the circuit so that in the case of a
line with several stations the talking or voice currents do not have
to flow through the generator armatures at the stations which are not
in use. Again, the normal shunting of the generator tends to save the
generator armature from injury by lightning.

[Illustration: Fig. 147. Circuit of Series Magneto Set.]

The more complete circuits of a series magneto telephone are shown in
Fig. 147. In this the line binding posts are shown as _1_ and _2_. At
the bottom of the telephone cabinet are four other binding posts
marked _3_, _4_, _5_, and _6_. Of these _3_ and _4_ serve for the
receiver terminals and _5_ and _6_ for the transmitter and battery
terminals. The circuits of this diagram will be found to be
essentially the same as those of Fig. 146, except that they are shown
in greater detail. This particular type of circuit is one commonly
employed where the generator, ringer, hook switch, and induction coil
are all mounted in a so-called magneto bell box at the top of the
instrument, and where the transmitter is mounted on an arm just below
this box, and the battery in a separate compartment below the
transmitter. The only wiring that has to be done between the bell box
and the other parts of the instrument in assembling the complete
telephone is to connect the receiver to the binding posts _3_ and _4_
and to connect the battery and transmitter circuit to the binding
posts _5_ and _6_.

_Bridging