Note:  Do not rely on this information. It is very old.


Key, in electricity, is an instrument used for opening and closing circuits conveying small currents. An elastic strip of brass is fixed to a base at one end, and has a platinum point and an insulating button at the end. By pressing the button the point can be put into contact with a fixed stud, also tipped with platinum. Wires are attached to the strip, and the studs can thus be connected with or disconnected from each other at pleasure. In some keys two fixed contact pieces are provided, one above and one below the strip; the latter can then be brought into connection with either, thus connecting one wire with either of two others. The Morse telegraphic key serves the same purpose, but has the two contacts not one above the other, but at opposite ends of a lever pivoted near the middle. Double contact keys are used in testing for charging and discharging condensers, etc. Reversing keys, used with single needle, mirror, and syphon recording telegraph instruments, have two double contact keys on the same base. The two top contacts are connected together and to one pole of a battery, the two bottom contacts being connected to the other pole; the two levers are connected to line and earth or to some instrument. When both levers are up the battery circuit is open, but a current may be sent in either direction by pressing one or other of the buttons. In plug keys a conical hole is drilled in a plate of brass, which is then divided by one or more saw cuts passing through rne centre of the hole; the pieces, being screwed to a base-board, may be connected or disconnected by inserting or withdrawing a taper plug which fits the hole.