Line Signal. The line signal in magneto switchboards is practically always in the form of an electromagnetic annunciator or drop.  It consists in an electromagnet adapted to be included in the line circuit, its armature controlling a latch, which serves to hold the drop or shutter or target in its raised position when the magnet is not energized, and to release the drop or shutter or target so as to permit the display of the signal when the magnet is energized.  The symbolic representation of such an electromagnetic drop is shown in Fig. 233.

[Illustration:  Fig. 233.  Drop Symbol]

Jacks and Plugs. Each line is also provided with a connection terminal in the form of a switch socket.  This assumes many forms, but always consists in a cylindrical opening behind which are arranged one or more spring 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.