tion: Fig. 188. Poole Lock-Out System]

Each of the relays has two windings, one of high resistance and the
other of low resistance. Remembering that the system to which this
device is applied is always a common-battery system, and that,
therefore, the normal condition of the line will be one in which there
is a difference of potential between the two limbs, it will be evident
that whenever any subscriber on a line that is not in use raises his
receiver from its hook, a circuit will be established from the upper
contact of the hook through the lever of the hook to the
high-resistance winding _1_ of the relay and thence to the other side
of the line by way of wire _6_. This will result in current passing
through the high-resistance winding of the relay and the relay will
pull up its armature. As soon as it does so it establishes two other
circuits by the closure of the relay armature against the contacts _4_
and _5_.

The closing of the contact _4_ establishes a circuit from the upper
side of the line through the upper contact of the switch hook, thence
through the contacts of the push button _3_, thence through the
low-resistance winding _2_ of the relay to the terminal _4_, thence
through the relay armature and the transmitter to the lower side of
the line. This low-resistance path across the line serves to hold the
relay armature attracted and also to furnish current to the
transmitter for talking. The establishment of this low-resistance path
across the line does another important thing, however; it practically
short-circuits the line with respect to all the high-resistance relay
windings, and thus prevents any of the other high-resistance relay
windings from receiving enough current to actuate them, should the
subscriber at any other station remove his receiver from the hook in
an attempt to listen in or to make a call while the line is in use. As
a subscriber can only establish the proper conditions for talking and
listening by the attraction of this relay armature at his station, it
is obvious that unless he can cause the pulling up of his relay
armature he can not place himself in communication with the line.

The second thing that is accomplished by the pulling up of the relay
armature is the closure of the contacts _5_, and that completes the
talking circuit through the condenser and receiver across the line in
an obvious fashion. The result of this arrangement is that it is the
first party who raises his re