er already explained in connection with Fig. 284. The third or
sleeve strand of the cord is continuous from plug to plug, and between
it and the ground there is permanently connected a retardation coil.

_Test._ The test is dependent on the presence or absence of a path to
ground from the test thimbles through some retardation coil associated
with a cord circuit. Obviously, in the case of an idle line there will
be no path to ground from the test thimbles, since normally they are
merely connected to each other and are insulated from everything else.
When, however, a plug is inserted into a multiple or answering jack, the
test thimbles of that line are connected to ground through the
retardation coil associated with the third strand of the plug used in
making the connection. When the operator applies the tip of the calling
plug to a test contact of a multiple jack there will be no path to
ground afforded if the line is idle, while if it is busy the potential
of the tip of the test plug will cause a current to flow to ground
through the impedance coil associated with the plug used in making the
connection. This will be made clearer by tracing the test circuit. With
the listening key thrown this may be traced from the live side of the
battery through the retardation coil _6_, which is common to an
operator's position, thence through the tip side of the listening key to
the tip conductor of the calling cord, and thence to the tip of the
calling plug and the thimble of the jack under test. If the line is idle
there will be no path to ground from this point and no click will
result, but if the line is busy, current will flow from the tip of the
test plug to the thimble of the jack tested, thence by the test wire in
the multiple to the thimble of the jack at which a connection already
exists, and thence to ground through the third strand of the cord used
in making that connection and the impedance coil associated therewith.
The current which flows in this test circuit changes momentarily the
potential of the tip side of the operator's telephone circuit, thus
unbalancing her talking circuit and causing a click.

[Illustration: Fig. 341. Magneto Multiple Switchboard]

If this test system were used in a very large board where the multiple
would extend through a great many sections, there would be some
liability of a false test due to the static capacity of the test
contacts and the test wire running through the multiple. For sma