2 Dr. 0. J. Lodge on a Form of Dcmiell Cell 



respect, that the materials remain always the same during 

 action except that the sulphate of zinc gradually increases in 

 quantity, a difference which scarcely affects the electromotive 

 force. Almost the only defect in the constancy of a cell so 

 charged is due to the fact that the two liquids diffuse into 

 each other, for which reason the battery cannot retain its 

 original state after it has stood for some time. Any thing 

 equivalent to a porous partition is quite useless for keeping the 

 liquids separate ; and the only plan seems to be to provide as 

 long a column of liquid as possible for the copper salt to dif- 

 fuse through. 



This is clone in a compact and simple manner in the cell 

 represented in fig. 1 (PL I.). . A wide-mouthed bottle (or a tall 

 jar) is fitted with a cork through which passes a wide glass tube 

 open at both ends. To the lower end of this tube a short 

 closed tube (like a test-tube) is tied with silk thread ; a long 

 strip of sheet zinc is put down the open tube ; and a copper 

 wire, recurved at the bottom and coated with sealing-wax ex- 

 cept at its two ends, is passed through the cork to the bottom 

 of the closed tube, where it is imbedded in a few crystals of 

 copper sulphate. The bottle is then nearly filled with dilute sul- 

 phate of zinc, and the cork with the tube is inserted, the latter 

 being so arranged that the end of the zinc strip and the mouth 

 of the short tube are both below the surface of the liquid. In 

 a short time a strong solution of sulphate of copper forms at 

 the bottom of the closed tube where the copper wire is bare, 

 and it gradually diffuses upward ; but in order to reach the 

 zinc it has to diffuse itself all through the water of the bottle 

 and then up the long tube containing the zinc ; and this takes 

 a long time, though it certainly does take place to some extent 

 in a week or so. 



But when I want to put the cell by for any length of time, 

 I pull the long tube a little higher up through the^ cork, so 

 that the mouth of the short tube emerges above the liquid and 

 thus entirely prevents diffusion. The zinc strip is also raised 

 out of the liquid by the same action. It is convenient to have 

 the cork fitting pretty air-tight ; or else evaporation may go 

 on from the edges of the tube, and the salts which crystallize 

 there may continue the diffusion slowly. 



The copper wire need not be covered with wax or any thing ; 

 but if it were not, its upper parts would assist in the action 

 until they were polarized ; and hence the internal resistance 

 would be liable to vary, which is not desirable. The internal 

 resistance of such a cell is always rather high : for instance, 

 in the one of which fig. 1 is a portrait, the bottle stands about 

 6 inches high, and the internal resistance is about 500 ohms 



