Definition of telegraph key in US English:

telegraph key

noun

  • A button which is pressed to produce a signal when transmitting Morse code.

    • ‘This was a five-key device based loosely on the telegraph key in which the transmission of information involved specific combinations of keys rather than the distinct meaning assigned to each key of the QWERTY typewriter keyboard.’
    • ‘Above each cylinder rests a fine platinum needle, or stylus, not unlike the point in a telegraph key.’
    • ‘With a clattering of telegraph keys, reporters sent news of the race to a world suddenly transfixed by the drama in the far north.’