THE ELECTRIC TELEGRAPH. 



In like manner, it may be shown that when the arm M is moved 

 from the notch 3 to the notch 4, the indicator at a' will be moved 

 from the horizontal position to one which will make an angle of 

 135, with its original direction, or what is the same, 45, with the 

 position in which it would point directly downwards. 



Without pursuing this explanation further, it will be easy to 

 see that the successive positions assumed by the hand of the 

 indicator at s' correspond with those given to the arm M of the 

 commutator at s. 



We have here explained the action of one commutator at s upon 

 one indicator at s'. The action of the other commutator at s upon 

 the other indicator at s' is precisely the same. It must be under- 

 stood, that the two commutators at s are connected with separate 

 and independent line-wires, are supplied with separate and inde- 

 pendent batteries, and act upon separate and independent indicators 

 at s'. The right-hand commutator at s is connected with the 

 right-hand indicator at s', and the left-hand commutator with the 

 left-hand indicator. 



From what has been explained, the process necessary, as well 

 for receiving as for transmitting a despatch will be understood. 

 In the reception of a despatch, the agent has only to place the 

 handle of his commutator in notch 1, and to see that his indicator 

 is vertical. After that he has only to observe the successive 

 attitudes assumed by the two indicators upon the dial before him, 

 and to write down the letters they successively express. 



Since this form of telegraph gives 64 signs, while 26 are suffi- 

 cient for the alphabet, and 10 for the numerals, there are 24 signs 

 disposable for abridgements, such as syllables, words, and phrases 

 of most frequent occurrence. 



187. The battery employed in working these telegraphs is at 

 present invariably that of Daniel (32). Formerly Bunsen's battery 

 (34) was used at chief stations, where great power is often required, 

 but this has now been discontinued. 



Between the point K' and the battery a commutator is placed, by 

 >ieans of which the agent can bring into action a greater or less 

 number of the pairs composing the battery, so as to proportion the 

 power to the distance to which the current is to be transmitted, or 

 to the resistance it may have to overcome. 



A perspective view of the telegraphic instrument, showing the 

 two indicators and two commutators, in their respective positions, 

 is given in fig. 72 (vol. iii. p. 193). 



FRENCH RAILWAY TELEGRAPH. 



188. The telegraphs which convey letters or words by conven- 

 tional signals, like those described above, require a staff of agents 



20 



