UATTEIUES. 



81 



Fio. 2. 



Tin' electromotive force of a Dauiell cell differs slightly 

 from one volt, and it is constant enough to be taken as a unit 

 in ordinary practice. 



The capital defect the only defect, we should say which 

 c.in be set against the Daniell cell, is that it consumes nearly 

 as much sulphate of copper and zinc in open circuit, when it 

 does not give off any useful work, as in closed circuit when it 

 is really used as a generator of electricity. 



BALLOON CELL. In order to avoid having to maintain the 

 sulphate of copper cell, some makers Breguet particularly 

 have arranged a glass flask, resting in an inverted position, on 

 the vessel of the cell (Fig. 2). This flask contains one kilo- 

 gramme of crystals of sulphate of copper, and is filled up with 

 water ; it is closed by means 

 of a cork stopper. The cork 

 stopper is pierced with a 

 hole, in which is fitted a 

 guttapercha tube, reaching 

 the liquid contained in the 

 porous vessel. The solution 

 of sulphate of copper being 

 the denser according to its 

 degree of concentration, the 

 bottom portion of the liquid 

 is always kept in a state of 

 saturation. As it becomes 

 weaker, it is replaced by the 

 saturated solution which 

 descends from the flask. 

 The external vessel itself is 

 closed by a wooden ring, so that the evaporation is very slight, 

 creeping salts are formed with difficulty, and the liquid lasts a 

 very long time. 



WILLIAM THOMSON CELL. The Daniell cell which has 

 been modified by Sir William Thomson in order to obtain 

 elements of a small internal resistance, is composed of wooden 

 trays lined with a sheet of lead ; on this lead rests the copper 

 strip which constitutes the negative electrode. At the four 



