THE ELECTRIC TELEGRAPH. 



indicator, which corresponds with it in form, is placed like the 

 dial of a clock in front of a vertical case. 



If we suppose the commutator (fig. 73) at the station s, and the 

 indicator at s', the arm of the commutator and that of the indi- 

 cator being upon the mark +> an y motion of tfye former made in 

 the direction of the hand of a clock, will produce a corresponding 

 motion of the hand of the latter, so that whatever letter or 

 number the one points to, the other will at the same time 

 point to. 



By this means the agent at s may spell word after word to the 

 agent at s'. 



There are various conventional signs, made by two or more 

 complete turns of the handle of the commutator, which, being 

 altogether arbitrary, and matters of local convenience, need not be 

 noticed here. 



It is found that moderately well-practised hands can transmit 

 with this instrument forty letters per minute, while the most 

 expert can send as many as sixty. 



A side view of the wheel-work and electro-magnet, E, of the 

 indicating apparatus is given in fig. 75. 



The armature, P, is alternately attracted and dismissed by the 



Fig. 75. 



magnet, acted on by the pulsations of the current, and imparts 

 this motion to the escapement at F, by which the hand A of the 

 indicator is advanced from letter to letter upon the dial, so that 

 the motion of the hand A at the station s' shall correspond exactly 

 with that of the hand of the commutator at the station s. 

 24 



