SCIENCE AND PRACTICE. 227 



The pulp is put loosely into the space between the porous 

 pot and glass jar, and then hammered down with wooden 

 tool and mallet, until it forms a compact mass. When the 

 element is filled to a sufficient height with pulp, an annular 

 disc of brown paper or cotton cloth is put over it. The zirc 

 ring z surrounds the tube c c, and rests upon the paper 

 disc e e, over the pulp, reaching to within an inch or so of 

 the top of the vessel. It is cast with a neck, g, which 

 prevents the local action that would set in between the 

 copper wire h and zinc, were the acidulated water to reach 

 the former. As it is, the whole ring is immersed in the 

 liquid ; the connection with it is made by means of a copper 

 terminal screw, ti, on the top of the wire h. 



To charge the element, small crystals of sulphate of 

 copper are dropped down the chimney c c, into the copper 

 compartment c' c ; both compartments are then filled to the 

 same height with water, which may be fresh, acidulated, or 

 contain table salt. 



When once set up, these elements require only to be 

 supplied with sulphate of copper from time to time, and the 

 water, which evaporates from the zinc compartment, re- 

 plenished. When new, these batteries have a greater 

 resistance than the common form of Daniell's, but not so 

 much as to incapacitate them from employment in local 

 circuits. Messrs. Siemens have during some years em- 

 ployed them to a great extent in measuring the insulation 

 resistances of cables, and have found them, when regularly 

 supplied with sulphate of copper and water, and cleaned 

 occasionally, to possess ninety per cent, of their original 

 electro -motive force at the end of six months. 



8. If, instead of paper-pulp, oxide of zinc be used for 

 forming the diaphragm, the zinc ring will remain for months 

 free from traces of copper, whether the element be in closed 

 or open circuit. This plan of employing oxide of zinc was 

 communicated to the author by Mr. Yarley, its inventor. 

 An element properly constructed upon this principle affords, 

 in our opinion, the best form of Daniell's element which 

 exists. The particles of foreign metals, separated from the 



Q2 



