62 ELECTROLYSIS. 



corners of these trays are placed small wooden blocks which 

 support a zinc grating, the latter being the positive electrode. 

 The cells are arranged in a column, as is the case with a 

 voltaic battery, this arrangement producing a powerful 

 battery in a comparatively limited space. The zinc is sur- 

 rounded by a sheet of parchment which acts as a porous vessel, 

 and prevents the mixture of the liquids. The battery is 

 charged with a solution of sulphate of zinc, to which is added 

 the sulphate of copper in small crystals and placed as regularly 

 as possible all around and at the bottom of the tray. 



CALLAUD CELL. The Callaud cell (Fig. 3) is a Daniell 

 cell without a porous jar, and in which the separation of the 

 liquids is obtained through their difference of density. It is 



FIG. 3. 



composed of a glass cylindrical jar, at the bottom of which a 

 ring made of a thin strip of copper is laid, and a cylinder of 

 zinc is suspended from the brim by means of three small hooks. 

 The solution of sulphate of copper being denser than that of 

 sulphate of zinc, keeps at the bottom of the jar, so that the two 

 electrodes are always dipping into their respective liquids 

 without any possibility of getting mixed. 



The copper rod which is used for suspending the cylinder of 

 copper is covered with guttapercha, without which precaution 

 experience has shown that it would in time part at the line of 

 separation of the two liquids. The addition of sulphate of 

 copper, when necessary, is effected by means of a siphon. 

 This cell is more particularly employed in the United States. 



