Page:Popular Science Monthly Volume 3.djvu/422

408 pallet moves back to the remote side, and the hand is advanced another letter. If the hand is initially at the cross, it will be advanced to any required letter by so arranging matters that the number of currents plus the number of interruptions shall be equal to the number denoting the place of the letter in the alphabet. To effect this arrangement is the office of the sending instrument.

This is represented in Fig. 7. There is a dial inscribed with 25 letters and a cross, like that of the receiving instrument, and an arm Fig. 8. Vibrating Alarum. which can be carried round the dial by a handle (M). There are 26 notches cut in the edge of the dial, in which a pin attached to the movable arm catches; and the arm is allowed sufficient play to and from the face of the dial to admit of this pin being easily released or inserted. When the pin is in one of the notches, the instrument is in position for transmitting the corresponding letter. The action is as follows: A toothed or rather undulated wheel is fixed on the same axis as the revolving arm, and turns with it. There are 13 projections and 13 hollows on its circumference, a few of which are shown in the figure where the face is cut away. A bent lever (T), movable about an axis at a, bears at one end against the circumference of the undulated wheel, while its other end plays between two points (P, Q), and is in contact with one or other of these points whenever its upper end bears against a hollow or a projection. P is in connection with a battery, and Q with the earth, the undulated wheel being in connection with the line-wire. The movement of the handle thus produces the requisite number of currents and interruptions.

Besides the sending and receiving apparatus above described, each station has an alarum, which is employed to call attention before sending a dispatch. There are several different kinds. Fig. 8 represents the vibrating alarum, which is one of the simplest. It contains an electro-magnet (e), with an armature (f), fixed to the end of an elastic plate. When no current is passing through the coil, the armature is held back by the elasticity of this plate, so as to press against a contact-spring (g) connected with the binding-screw (m). The terminals of the coil are at the binding-screws (p, p'), the former of which is in connection with the armature, and the latter with the earth. As long as the armature presses against the spring (g), there is communication between the two binding-screws (m and p') through the coil; but the passing of a current produces attraction of the armature, which draws