Page:Heroes of the telegraph (IA cu31924031222494).djvu/284

 marking time, giving signals, and printing intelligence at different places. He also proposed to utilise 'natural bodies of water' for a return wire, but the earlier experimenters had done so, particularly Steinheil in 1838. The most important idea in the patent is, perhaps, his plan for inverting the needle telegraph of Ampère, Wheatstone and others, and instead of making the signals by the movements of a pivoted magnetic needle under the influence of an electrified coil, obtaining them by suspending a movable coil traversed by the current, between the poles of a fixed magnet, as in the later siphon recorder of Sir William Thomson. Bain also proposed to make the coil record the message by printing it in type; and he developed the idea in a subsequent patent.

Next year, on December 31st, 1844, he projected a mode of measuring the speed of ships by vanes revolving in the water and indicating their speed on deck by means of the current. In the same specification he described a way of sounding the sea by an electric circuit of wires, and of giving an alarm when the temperature of a ship's hold reached a certain degree. The last device is the well-known fire-alarm in which the mercury of a thermometer completes an electric circuit, when it rises to a particular point of the tube, and thus actuates an electric bell or other alarm.

On December 12, 1846, Bain, who was staying in Edinburgh at that time, patented his greatest invention, the chemical telegraph, which bears his name. He recognised that the Morse and other telegraph instruments in use were comparatively slow in speed, owing to the mechanical inertia of the parts; and he saw that if the signal currents were made to pass through a band of travelling paper soaked in a solution which would decompose under their action, and leave a legible mark, a very high speed could be obtained. The chemical he employed to saturate the paper was a solution of nitrate of ammonia and prussiate of potash, which left a blue stain on being decomposed by the current from an iron contact or stylus. The signals were the short and long, or 'dots' and 'dashes' of the Morse code. The speed of marking was so great that hand signalling could not keep up with it, and Bain devised a plan of automatic signalling by means of a running band of paper on which the signals of the message were represented by holes punched through it. Obviously if this tape were passed between the contact of a signalling key the current would merely flow when the perforations