Fig. 235. Plug
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Fig. 236. Plug and Jack
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Fig. 237. Jack and Plug Symbols
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Keys. Other important elements of manual switchboards are ringing and listening keys. These are the devices by means of which the operator may switch the central-office generator or her telephone set into or out of the circuit of the connected lines. The details of a simple ringing and listening key are shown in Fig. 238. This consists of two groups of springs, one of four and one of six, the springs in each group being insulated from each other at their points of mounting. Two of these springs 1 and 2 in one group—the ringing group—are longer than the others, and act as movable levers engaging the inner pair of springs 3 and 4 when in their normal positions, and the outer pair 5 and 6 when forced into their alternate positions. Movement is imparted to these springs by the action of a cam which is mounted on a lever, manipulated by the operator. When this lever is moved in one direction the cam presses the two springs 1 and 2 apart, thus causing them to disengage the springs 3 and 4 and to engage the springs 5 and 6.