CHAP, in.] ELECTRICITY AND MAGNETISM. 149 



used in telegraphy, water only is supplied at the outset 

 in the cells containing the zincs ; and the E.M.F. of these 

 is less than if acid or a solution of sulphate of zinc were 

 added to the water. 



182. Other Batteries. Numerous other forms of 

 battery have been suggested by different electricians. 

 There are three, of theoretical interest only, in which 

 the electromotive -force is due, not to differences of 

 potential at the contact of dissimilar metals, but to differ- 

 ences of potential at the contact of a metal or metals 

 with liquids. The first of these was invented by the 

 Emperor Napoleon III. Both plates were of copper, 

 dipping respectively into solutions of dilute sulphuric 

 acid and of cyanide of potassium, separated by a porous 

 cell. The second of these combinations, due to Wohler, 

 employs plates of aluminium only, dipping respectively 

 into strong nitric acid and a solution of caustic soda. 

 In the third, invented by Dr. Fleming, the two liquids 

 do not even touch one another, being joined together by 

 a second metal. In this case the liquids chosen are 

 sodium persulphide and nitric acid, and the two metals 

 copper and lead. A similar battery might be made with 

 copper and zinc, using solutions of ordinary sodium, 

 sulphide, and dilute sulphuric acid in alternate cells, a 

 bent zinc plate dipping into the first and second cells, a 

 bent copper plate dipping into second and third, and so 

 on ; for the electromotive -force of a copper-sodium 

 sulphide-zinc combination is in the reverse direction to 

 that of a copper-sulphuric acid-zinc combination. 



Jablochkoff has described a battery in which plates of 

 carbon and iron are placed in fused nitre ; the carbon is 

 here the electro-positive element, being rapidly consumed 

 in the liquid. It furnishes powerful currents. 



Planters and Faure's Secondary Batteries, and Grove's 

 Gas Battery, are described in Lesson XXXVIII. 



The so-called Dry Pile of Zamboni deserves notice. 

 It consists of a number of paper discs, coated with zinc- 



