653 



[Barker. 



means of a tubukire in the bend this tube was filled with the zinc sul- 

 phate solution. When not in use, the U tube is removed and kept in a 

 separate vessel. Kempe in 1880 {J. 8oc. Teleg. Eiuj., June, 1880), described 

 a standard Daniell cell which has been adopted in the British Post-Office. 

 The containing vessel is of porcelain, having two compartments. In one 

 of these is a half saturated solution of zinc sulphate, reaching to the lower 

 edge of the zinc plate. In the other is a flat, porous cup containing the cop- 

 per plate surrounded with crystals of copper sulphate, and immersed in 

 copper sulphate solution. To use this battery, the porous cup is transferred 

 from one compartment to the other, thus raising the zinc solution into 

 contact with the zinc plate. After miking the measurement, the porous 

 cup is replaced in its own compartment. Any copper which may have 

 been carried into the zinc solution is precipitated upon a fragment of zinc 

 kept constantly in it. 



Having had occasion for a series of months, at intervals, to make measure- 

 ments of electromotive force by the method of comparison, I have been led 

 to devise a form of standard Daniell cell which appears to have so important 

 advantages over others heretofore used as to justify me in bringing it to the 

 notice of the Society. The form of apparatus which has been adopted is 

 represented in the annexed wood cut. It consists of two bottles with lateral 

 tubulures near the bottom. These are closed with rubber corks through 

 which passes a stop-cock of glass. The necks of the bottles also 

 carry corks of rubber, through which piss the rods of zinc and copper. 

 The bottle containing the rod of zinc is filled about three-fourths with a 

 solution of zinc sulphate saturated at 15^ C. That containing the copper 

 rod with a saturated solution of copper sulphate. When the cell is to be 

 used for measurement, the cock is opened and the two liquids are thus put 

 in communication. At the end of the experiment, it is again closed and 

 all diffusion is prevented.* For ordinary use, especially where a large 

 number of cells in series is required, a much cheaper apparatus may be 

 constructed. Those set up in my own laboratory consist of a couple of 

 the cheap bottles now in general use for the nasal douche and for contain- 

 ing dry plate developers, which have a small lateral spout at the bottom. 

 Over these a rubber tul)e may be passed and tied, being closed when re- 

 quired by a wire compressor. In practice I have found it an advantage to 

 place a wisp of spun glass in the rubber tube to prevent adherence be- 

 tween its sides. The zinc and the copper rods pass through corks in the 

 mouths of the bottles as before. 

 Tlie advantages which are claimed for this new form of cell are : 

 1st. Its constancy. "When set up, all such cells are identical. The zinc 

 is in contact with a solution of zinc sulphate, and the copper with one 

 of copper sulphate both saturated at 15° C. Moreover, this iden- 

 tity continues. When on closed circuit, the liquids are altered by dif- 

 fusion to a scarcely appreciable extent, the surface of contact being so 

 small. During action copper sulphate is decomposed on one side and cop- 



Tbe cell here represented was made for me by J. W. Qneen & Co., of this city. 



