CHAP. V.] 



TELEGRAPHIC LINES. 



621 



Marie Davy elements working night and day over a line of 500 

 kilometres has furnished a current of sufficient constancy for a 

 period of nearly four months. 



The number of electro -motive apparatus invented for the service of 

 electric telegraphy is so considerable that space would fail us to name 



FIG. 40.5. Darnell's battery employed in telegraphy. 



them, much less to describe them. Many among this number are 

 remarkable for certain particular qualities and have been used with 

 success. It is easy however to understand that the success of their 

 employment depends on the apparatus for which the battery is 

 destined, according as it requires greater or less electro-motive force. 

 We may add, in conclusion, that a distinction must be made 

 between the batteries for the line, which send currents to o-reat dis- 



Fio. 406. Marie Davy's sulphate of mercury battery. 



tances, and the local batteries which have only to serve the apparatus 

 of the station itself. These last, whose circuit is very short and which 

 have not to furnish electricity to the line, are formed of a small number 

 of elements whose total electro-motive force is naturally very inferior 

 to that of the line batteries. 



