Abstract
THE speaking of the telephone is admittedly so weak that it can only be caught by keeping the instrument in immediate contact with the ear. Hence there is transmitted through the telephone in its present form no sound which would be intense enough to announce to any one who was in a large room and who did not hold the telephone close to his ear, that a message was about to be sent from the transmitting station. The consequence is that a warning apparatus must be attached to the telephone, so that there may be no fear of missing a projected telephonic conversation.
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RÖNTGEN, W. A Telephonic Alarum . Nature 17, 164 (1877). https://doi.org/10.1038/017164a0
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DOI: https://doi.org/10.1038/017164a0