e left-hand key. Then, obtaining an operator beyond the break by any
available means, he instructs him likewise to insert plugs _1_ and _2_
in jacks _9_ and _2_, and plugs _3_ and _4_ in jacks _11_ and _4_,
similarly throwing the left-hand key. By tracing this out, it will be
observed that the train wire is patched over the disabled section by
means of the message circuit, and that the selector and the telephone
equipment are cut over on to the patched connections; in other words,
bridged across the patching cords.

[Illustration: Fig. 497. Circuits of Test Board]

It will also be seen that with this board it is possible to open any
circuit merely by plugging into a jack. Two wires can be short-circuited
or a loop made by plugging two cords of corresponding colors into the
two jacks. A ground jack is provided for grounding any wire. In this
way, a very flexible arrangement of circuits is obtained, and it is
possible to make any of the simple tests which are all that are usually
required on this type of circuit.

=Blocking Sets.= As was just mentioned, quite frequently in addition to
train wires and message circuits, block wires are also operated by
telephone. In some cases separate telephone instruments are used for the
blocking service, but in others the same man handles all three circuits
over the same telephone. The block wire is generally a converted
telegraph wire between stations, usually of iron and usually grounded.
It seldom ranges in length over six miles.

[Illustration: Fig. 498. Blocking Set]

Where the block wires are operated as individual units with their own
instruments, it is unnecessary to have any auxiliary apparatus to be
used in connection with them. Where, however, they are operated as part
of a system and the same telephone is used on these that is used on the
train wire and message wire, additional apparatus, called a blocking
set, is required. This blocking set, shown in Figs. 498 and 499, was
developed especially for this service by the Western Electric Company.
As will be noted, a repeating coil at the top and a key on the front of
the set are wired in connection with a pair of train wire cords. This
repeating coil is for use in connecting a grounded circuit to a metallic
circuit, as, for instance, connecting a block wire to the train wire,
and is, of course, for the purpose of eliminating noise. Below the key
are three combined jacks and signals. One block wire comes into each of
these