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Wireless Telegraphy 9 that position even when no current was flowing. Consequently it became necessary to break the bridge after the wave had passed. This was a simple matter. It was only necessary to give the tube a sharp, slight jar or knock to induce the particles to fall apart. In the earliest days this was done by striking the tube sharply with the finger. Later, to facilitate the opera- tion, a small hammer-like device was introduced which, striking the tube after the wave had passed, broke the bridge. This tiny hammer was operated by clockwork mechanism, and from its function became known as the “ tapper-back,” because it reduced the dust particles to their original independent condition, or “ decohered ” them, as it was termed. In 1895 another far-reaching improvement was made by the perfection of an automatic tapper-back by Professor A. Popoff, of the Russian Torpedo School at Cronstadt. But here again an invention was de- vised for a purpose quite away from wireless tele- graphy. This scientist was deeply engaged in the study of lightning phenomena, and he evolved an interesting instrument for observing these electrical discharges. He took the coherer and combined with it a relay, recording mechanism, and the tapper-back. By means of this ingenious instrument he was able to record automatically upon a strip of paper the time of each lightning flash. More than this was rendered possible. After a flash had been received and recorded the instrument automatically reset itself for recording the next flash. The relay actuated the tapper-back, which, striking the tube, decohered the particles. This ingenious instrument successfully solved a problem which had been perplexing several