this will be obvious from the following
consideration. When a call originates on any line, either grounded or
metallic, the operator does not know which kind of a line is to be
called for. She, therefore, plugs into this line with any one of her
answering plugs and completes the connection in the usual way. If the
call is for the same kind of a circuit as that over which the call
originated, she places the converting key in such a position as will
connect the conductors of the cord circuit straight through; while if
the connection is for a different kind of a line than that on which
the call originated she throws the converting key into such a
position as to include the repeating coil. A study of Fig. 280 will
show that when the converting key, which is commonly referred to as
the repeating-coil key, is in one position, the cord conductors will
be cut straight through, the repeating coil being left open in both
its windings; and when it is thrown to its other position, the
connection between the answering and calling sides of the cord circuit
will be severed and the repeating coil inserted so as to bring about
the same effects and circuit arrangements as are shown in Fig. 279.

[Illustration: Fig. 280. Convertible Cord Circuit]

Cord-Circuit Considerations. _Simple Bridging Drop Type._ The matter
of cord circuits in magneto switchboards is deserving of much
attention. So far as talking requirements are concerned, the ordinary
form of cord circuit with a clearing-out drop bridged across the two
strands is adequate for nearly all conditions except those where a
grounded-and a metallic-circuit line are connected together, in which
case the inclusion of a repeating coil has some advantages.

[Illustration: Fig. 281. Bridging Drop-Cord Circuit]

From the standpoint of signaling, however, this type of cord circuit
has some disadvantages under certain conditions. In order to simplify
the discussion of this and other cord-circuit matters, reference will
be made to some diagrams from which the ringing and listening keys and
talking apparatus have been entirely omitted. In Fig. 281 the regular
bridging type of clearing-out drop-cord circuit is shown, this being
the type already discussed as standard. For ordinary practice it is
all right. Certain difficulties are experienced with it, however,
where lines of various lengths and various types of sub-station
apparatus are connected. For instance, if a long bridging line be
conn