Page:Popular Science Monthly Volume 3.djvu/420

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place. In the Morse alphabet, which is now the telegraphic alphabet of all nations, the shortest signs are allotted to those letters which occur most frequently. This was not the case with the old needle-alphabet, which was rather planned with the view of assisting the memory; and experience has shown that such assistance is quite unnecessary. The needle instrument is also, to a great extent, being superseded by Morse's instrument.

Telegraphs in which the ordinary letters of the alphabet are ranged round the circumference of a dial, and are pointed at by a revolving hand, are specially convenient for those who are not professional telegraphists. They are constructed on the principle of step-by-step motion, the hand being advanced by successive steps, each representing one current sent or stopped.

One of the simplest instruments of this class is Breguet's, which is extensively used on the French railways. Fig. 4 represents the exterior of the receiving instrument. The dial is inscribed with the 25 letters of the French alphabet and a cross, making 26 signals in all. The hand (as in other step-by-step telegraphs) advances only in one direction, which is the same as that of the hands of a clock, stopping before each letter which is to be indicated, and pointing to the cross at the end of each word. Fig. 5 shows the mechanism by which the

Fig. 5.
PSM V03 D420 Mechanism of indicator.jpg
Mechanism of Indicator.

motion is produced. A is the armature of an electro-magnet, the magnet itself being removed in the figure to allow the other parts to be better seen. The two dotted circles traced on the armature represent vertical sections of the two coils, which rest on the bottom of the box, and have their axes horizontal. If introduced, they would nearly conceal