2S2 



ELECTRO-TELEGRAPHY 



803 



804 



spaces between them being filled with sand moistened with dilute sulphuric acid. 

 These batteries are usually arranged in boxes of one dozen elements; they remain 

 tolerably constant for some weeks without attention and are readily recharged. 



The Marie-Davy battery is much used in Franco. A zinc plate is immersed in pure 

 water in practice the zinc cylinders are packed in wet sponge and carbon plates in 

 porous cylinders are immersed in a paste of proto-sul- 

 phato of mercury and water. This battery requires very 

 little attention, and it is said to retain its original force 

 for six months. 



Meidinger produced a modification of Daniell's battery, 

 which was charged with a solution of Epsom salts and 

 sulphate of copper. This has, however, given way to the 

 battery of Siemens and Halske, in which the peculiarity 

 is the use of paper-pulp, in addition to the porous 

 earthenware diaphragm, to prevent the mixing of thefluids 

 too readily. Into a glass jar A A., fig. 803 is placed a cross- 

 shaped plate of copper c, united to a copper-wire c, which 

 rises above the jar. Over this is placed a porous earthen- 

 ware cylinder b b, widened out at the bottom into a boll 

 shape. Between this porous bell and the glass jar. paper- 

 pulp D, saturated with a quarter of its weight of sul- 

 phuric acid, which converts it into a gelatinous mass, 

 and then worked up with four times its weight of water 

 is pressed so as to form a compact mass ; this about 

 half fills the jar, and upon it is placed an annular disc 

 of brown paper or of cotton cloth E E. The zinc cylinder 

 F F, cast with a neck which rises above the fluid, rests upon this disc, reaching to 

 within an inch of the top of the jar. Crystals of sulphate of copper are dropped 



into the porous cylinder, 

 and then both compart- 

 ments are filled to the 

 same height with water, 

 which may be fresh or 

 acidulated. This bat- 

 tery only requires to bo 

 supplied with sulphate 

 of copper from time to 

 time, and water added 

 to supply the waste by 

 evaporation. It has 

 been found that at the 

 end of six months the 

 electro-motive force was 

 ninety per cent, of what 

 it was originally. 



Under whatever cir- 

 cumstances a wire takes 

 part in promoting the 

 discharge of a voltaic 

 apparatus, the whole 

 of the said wire is in 

 a condition to indicate 

 the presence of the force 

 that is pervading it; and 

 as the force may bo pre- 

 sented to the wire in 

 either of two directions, 

 that is to say, the copper 

 or the zinc, namely, the 

 positive or the negative 

 end of the battery, may 

 bo presented to the 

 given end of the tele- 

 graph wire, the relative 

 condition of the wire will bo modified accordingly. Not only can the direc- 

 tion of this current-force be inverted at pleasure, but it can be maintained 1 

 length of time, great or small, and in either direction. This is accomplished by 



