408 



THE POPULAR SCIENCE MONTHLY. 



Vibrating Alarum. 



pallet moves back to the remote side, and the hand is advanced an- 

 other 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 num- 

 ber 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. V. There is a dial inscribed with 25 let- 

 ters and a cross, like that of the receiving instrument, and an arm 



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 piu 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 requi- 

 site 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 send- 

 ing 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 con- 

 tact-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 andp') through the coil ; but the 

 passing of a current produces attraction of the armature, which draws 



