52 LOCALIZED ELECTRIZATION. 



a considerable quantity of the crystals, and supplying the action 

 of the battery for a long period. 



For more than ten years, I have had constantly ia action, in 

 my room, twenty or twenty-five Daniell's elements of this arrange- 

 ment, some to charge my apparatus, others to work a telegraph, 

 bells, and an electric clock. To these, when united, I add at 

 pleasure a score of other elements of the same kind, which I have 

 used since 1860, for some new investigati(ms of the action of con- 

 tinuous constant currents ; investigations, the results of which will 

 be stated in Chapter III. 



Well, notwithstanding the greatest care, this pile is far from 

 being constant ; and the least change of temperature is sufficient 

 to cause considerable changes in the force of its current between 

 one day and the next. These variations of temperature increase 

 or diminish the concentration of the saline solutions, whence result 

 proportionate variations in the strength of the current. To avoid 

 these considerable oscillations, it is necessary that the room in 

 which the pile is placed should be constantly maintained at the 

 same temperature, by an appropriate system of warming. This 

 precaution is indispensable in winter, because a frost will diminish 

 the power of the apparatus by one-fourth, one-half, or even more. 

 Changes of temperature also occasion damage and accidents to the 

 pile, which becomes encrusted, and the porous vases are broken 

 by the crystallization that takes place in their pores. In summer, 

 a Daniell's battery is always more powerful, because the saline 

 solutions are more concentrated ; but it is not the less exposed to 

 considerable oscillations, only not so considerable as in the winter. 

 It is said that a Danit?irs balloon battery will act for six months ; 

 but, under the most favourable circumstances, I have always cleaned 

 mine at least every three weeks. It requires experience to know 

 how dirty and tedious is that manipulation ; and it will be found 

 necessary, on each occasion, to replace a greater or less number 

 of the porous cells, and of the balloons, so that the maintenance of 

 the apparatus becomes somewhat costly. 



(e). Marie-Davy s hattery of protosuljyhate of mercury. The de- 

 sideratum that I have pointed out appears to me to be fulfilled, 

 in great measure, by the protosulphate of mercury battery of M. 

 Marie-Davy (fig. 13).^ This battery has been used at the Ministry 

 of the Interior, for the service of the electric telegraph, for a period 

 of eight months, requiring no other care during that time, than 

 an occasional addition of water to supply the loss by evaporation. 

 Tho current appears to have been, in this application to telegraphy, 



' M. Marie-Davy presented his battery to the Academy of Sciences in 1854. 



.i 



