Key.
A switch adapted for making and breaking contact easily when worked by
hand, as a Morse telegraph key.
Key Board.
A board or tablet on which keys or switches are mounted.
Key-board.
(a) A switch board, q. v.
(b) A set of lettered keys similar to those of a typewriter employed in
some telegraph instruments. As each key is depressed it produces the
contact or break requisite for the sending of the signal corresponding
to the letter marked upon the key. The signal in printing telegraphs, on
which such key-boards are used, is the reprinting of the letter at the
distant end of the line.
Key, Bridge.
A key for use with a Wheatstone Bridge, q.v. It is desirable to first
send a current through the four arms of the bridge in using it for
testing resistances and then through the galvanometer, because it takes
a definite time for the current to reach its full strength. This is
especially the case if the element being measured has high static
capacity, as a long ocean cable. If the galvanometer connections were
completed simultaneously with the bridge connections a momentary swing
would be produced even if the arms bore the proper relation to each
other. This would cause delay in the testing. A bridge key avoids this
by first connecting the battery circuit through the arms of the bridge,
and then as it is still further depressed the galvanometer circuit is
completed.