In Fig. 146 are shown, in simplified form, the circuits of an ordinary series telephone.  The receiver in this is shown as being removed from the hook and thus the talking apparatus is brought into play.  The line wires 1 and 2 connect respectively to the binding posts 3 and 4 which form the terminals of the instrument.  When the hook is up, the circuit between the binding posts 3 and 4 includes the receiver and the secondary winding of the induction coil, together with one of the upper contacts 5 of the switch hook and the hook lever itself.  This completes the circuit for receiving speech.  The hook switch is provided with another upper contact 6, between which and the contact 5 is connected the local circuit containing the transmitter, the battery, and the primary of the induction coil in series.  The primary and the secondary windings are connected together at one end and connected with the switch contact 5, as shown.  It is thus seen that when the hook is up the circuit through the receiver is automatically closed and also the local circuit containing the primary, the battery, and the transmitter.  Thus, all the conditions for transmitting and receiving speech are fulfilled.

[Fig. 146.  Circuit of Series Magneto Set]

When the hook is 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.]