ies or bridge relation with the telephone line when the hook is
down.

_Series._ In the so-called series telephone line, where several
telephones are placed in series in a single line circuit, the
employment of the series type of telephone results in all of the
telephone bells being in series in the line circuit. This means that
the voice currents originating in the telephones that are in use at a
given time must pass in series through the magnets of the bells of the
stations that are not in use. In order that these magnets, through
which the voice currents must pass, may interfere to as small a degree
as possible with the voice currents, it is common to employ
low-resistance magnets in series telephones, these magnets being wound
with comparatively few turns and on rather short cores so that the
impedance will be as small as possible. Likewise, since the generators
are required to ring all of the bells in series, they need not have a
large current output, but must have sufficient voltage to ring through
all of the bells in series and through the resistance of the line. For
this reason the generators are usually of the three-bar type and
sometimes have only two bars.

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