Page:Popular Science Monthly Volume 3.djvu/423

Rh it away from g, and interrupts the current. The electro-magnet is thus demagnetized, and the armature springs back against g, so as to allow a fresh current to pass. The armature is thus kept in continual vibration; and a hammer (K), which it carries above, produces repeated strokes on a bell (T).

Morse's apparatus, first tried in America about 1837, is now perhaps the most extensively used of all.

His receiving instrument, or indicator, in its primitive simplicity, consists (Fig. 9) of an electro-magnet, a lever movable about an axis, carrying a soft-iron armature at one end, and a pencil at the other, and a strip of paper which is drawn past the pencil by a pair of rollers.



As the pencil soon became blunt, and was uncertain in its marking a point, which scratched the paper, was substituted. This has now, to a great extent, been superseded by an ink-writer, which requires the exertion of less force, and at the same time leaves a more visible trace.

Fig. 10 represents Morse's indicator as modified by Digney. A train of clock-work, not shown in the figure, drives one pair of rollers (n m), which draw forward a strip of paper (p p) forming part of a long roll (K). The same train turns the printing-cylinder (H), the surface of which is kept constantly charged with a thick, greasy ink by rolling-contact with the ink-pad (L). The armature (B B') of the electro-magnet (A) is mounted on an axis at C, and carries a style at its extremity just beneath the printing-cylinder. When a current passes, the armature is attracted, and the style presses the paper against the printing-cylinder, causing a line to be printed on it, the length of which depends on the duration of the current, as the paper continues to advance without interruption. The lines actually