ing contacts. The opening forms a receptacle for plugs
which have one or more metallic terminals for the conductors in the
flexible cord in which the plug terminates. The arrangement is such
that when a plug is inserted into a jack the contacts on the plug will
register with certain of the contacts in the jack and thus continue
the line conductors, which terminate in the jack contacts, to the cord
conductors, which terminate in the plug contacts. Usually also when a
plug is inserted certain of the spring contacts in the jack are made
to engage with or disengage other contacts in the jack so as to make
or break auxiliary circuits.

[Illustration: Fig. 234. Spring Jack]

A simple form of spring jack is shown in section in Fig. 234. In Fig.
235 is shown a sectional view of a plug adapted to co-operate with
the jack of Fig. 234. In Fig. 236 the plug is shown inserted into the
jack. The cylindrical portion of the jack is commonly called the
_sleeve_ or _thimble_ and it usually forms one of the main terminals
of the jack; the spring, forming the other principal terminal, is
called the _tip spring_, since it engages the tip of the plug. The tip
spring usually rests on another contact which may be termed the
_anvil_. When the plug is inserted into the jack as shown in Fig. 236,
the tip spring is raised from contact with this anvil and thus breaks
the circuit leading through it. It will be understood that spring
jacks are not limited to three contacts such as shown in these figures
nor are plugs limited to two contacts. Sometimes the plugs have three,
and even more, contacts, and frequently the jacks corresponding to
such plugs have not only a contact spring adapted to register with
each of the contacts of the plug, but several other auxiliary contacts
also, which will be made or broken according to whether the plug is
inserted or withdrawn from the jack. Symbolic representations of plugs
and jacks are shown in Fig. 237. These are employed in diagrammatic
representations of circuits and are supposed to represent the
essential elements of the plugs and jacks in such a way as to be
suggestive of their operation. It will be understood that such symbols
may be greatly modified to express the various peculiarities of the
plugs and jacks which they represent.

[Illustration: Fig. 235. Plug]

[Illustration: Fig. 236. Plug and Jack]

[Illustration: Fig. 237. Jack and Plug Symbols]

_Keys_. Other important elements of manual switc