CHAP, in.] ELECTRIC TELEGRAPHY. 571 



the twenty-six teeth of the wheel R, and consequently the advance by 

 one division of the dial needle, which is mounted on the same axis as 

 the wheel. 



In Froment's dial telegraph, Fig. 370, the indicator differs in no 

 respect from that of Breguet's. But the manipulator is distinguished 

 by a particular method of transmitting and interrupting the currents. 

 In this instrument an undulating groove determines by its rotation 

 the oscillations of the lever A, one of whose branches works in it. 

 It is easily understood then, without further detail, how the other 

 branch of the oscillating lever serves to commence and interrupt the 

 successive currents. What requires explanation is the manner in 

 which M. Froment has arranged this transmission of motion so that 

 the number of current emissions may be that which corresponds to the 

 position of each signal on the dial. 



FIG. 369. Indicator of Siemens' and Halske's telegraph. 



A clockwork arrangement moves the wheel B. But for this move- 

 ment to take place the tooth on the circumference of that wheel must 

 be disengaged from the catch E by which it is held in place. This 

 disengagement is accomplished by the action of a key-board, each of 

 whose keys corresponds to a letter or a number. By depressing one 

 of these keys the catch is moved by a bar which raises it, and the 

 rotation of the wheel commences under the influence of the clock- 

 work with a velocity of two or three turns a second. Below the key- 

 board is a metal axle, or cylinder D E, which turns with the wheel B, 

 and on the same axis. This axis has as many pegs as there are keys, 

 forming two series arranged in spirals ; each peg corresponds to one of 

 the keys, and its angular position on the cylinder depends on the 



