620 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



III. THE BATTERIES EMPLOYED IN TELEGRAPHY. 



The various systems of telegraphs we have described, with those 

 we have only mentioned, may be divided, as regards their electro- 

 motive power into two classes ; the first comprising the apparatus 

 which make use of a constant battery, and the second, those whose 

 power is derived from magneto-electric induction machines. 



A whole chapter in the Forces of Nature was devoted to 

 batteries : but it is well, nevertheless, to return to this subject 

 from the exclusive point of view of their application to telegraphy. 



The old batteries of Bunsen and Daniell were the first made use 

 of, and the second is still generally employed in France, while 

 Bunsen's battery is only used on certain American lines. In 

 England the electric telegraph is served by trough batteries, the 

 compartments of which are filled with sand impregnated with a 

 solution of ammonia hydrochlorate or acidulated water, with a plate 

 of amalgamated zinc and one of copper, in each compartment. This 

 battery furnishes a current of little intensity, bub one which is 

 especially suitable for the systems of needle telegraphs. 



Daniell's battery is easy to keep in order. It only requires a 

 little liquid poured in from time to time to repair the losses from 

 evaporation in the vessel containing the acidulated water and in the 

 porous vessel containing the solution of sulphate of copper, and to 

 see that the crystals of sulphate are always in sufficient quantity. 

 The crystalline efflorescence, also, that is deposited on the sides must 

 be removed from time to time, and the zinc plates replaced when 

 the amalgamation is destroyed. The constancy of the current of this 

 battery, which can work for nearly three months without trouble on 

 a line so much used, and so long as that from Paris to Berlin, 

 makes it a good electromotor. The number of Daniell's elements 

 employed for distances of 100, 200, 400 kilometres is 30, 50, 

 and 70. 



The Marie Davy sulphate of mercury battery is also employed in 

 France. The original arrangement of this element (Fig. 404) has 

 been replaced by a form analogous to that of Daniell's couple with 

 a cylinder of carbon instead of a sheet of copper, A battery of 38 



