Sir William Thomson's siphon recorder, in all its present completeness, must take rank as a masterpiece of invention. As used in the recording or writing in permanent characters of the messages sent through long submarine cables, it is the acknowledged chief of 'receiving instruments,' as those apparatus are called which interpret the electrical condition of the telegraph wire into intelligible signals. Like other mechanical creations, no doubt its growth in idea and translation into material fact was a step-by-step process of evolution, culminating at last in its great fitness and beauty.
The marvellous development of telegraphy within the last generation has called into existence a great variety of receiving instruments, each admirable in its way. The Hughes, or the Stock Exchange instruments, for instance, print the message in Roman characters; the sounders strike it out on stops or bells of different tone; the needle instruments indicate it by oscillations of their needles; the Morse daubs it in ink on paper, or embosses it by a hard style; while Bain's electro-chemical receiver stains it on chemically prepared paper. The Meyer-Baudot and the Quadruple receive four messages at once and record them separately; while the harmonic telegraph of Elisha Gray can receive as many as eight simultaneously, by means of notes excited by the current in eight separate tuning forks.
But all these instruments have one great drawback for delicate work, and, however suitable they may be for land lines, they are next to useless for long cables. They require a certain definite strength of current to work them, whatever it may be, and in general it is very considerable. Most of the moving parts of the mechanism are comparatively heavy, and unless the current is of the proper strength to move them,
