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. 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.