operating only on reception of predetermined code signals, e.g. distress signals, party-line call signals

Full Title

Apparatus or local circuits for transmitting or receiving dot-and-dash codes, e.g. Morse code (teaching apparatus therefor G09B; telegraph tapping keys H01H21/86) > Apparatus or circuits at the receiving end > operating only on reception of predetermined code signals, e.g. distress signals, party-line call signals