688 MEIDINGER'S ELEMENTS. 



is in the place where it is to remain afterwards ; if balloon and cell 

 together are carried about from place to place, the liquids will 

 inevitably mix, and the action of the cell is disturbed. If possible 

 the battery should be kept in a room in which no fire is lit, and in 

 which the temperature does not change very rapidly. If the 

 elements are allowed to stand unused, some of the solution of cupric 

 sulphate rises in consequence of diffusion (see page 215), and 

 reaches the zinc cylinder. Zinc decomposes the copper sulphate, 

 zinc sulphate and copper being formed, the latter being deposited 

 as a soft brownish-black mass, which interferes with the action of 

 the cell when it is to be used. But if the current is closed, either 

 permanently, or at least from time to time, the ascent of the solution 

 of cupric sulphate is prevented by the action of the current which 

 constantly carries it to the cathode, that is, downwards to the 

 copper. A battery of Meidinger elements should therefore, if not 

 in use, be closed from time to time for a few hours. 



Fig. 349 C represents in section a modification of a Meidinger 

 element which the student may construct for himself. Procure a 

 suitable glass jar, and let a tinsmith make you a cylinder of zinc, 2 

 or 3 mm thick, about l cm less in width than your jar, and about half 

 as high. Drill three holes into the cylinder, near the edge, at 

 equal distances from one another, and each about 1*5 or 2 mm wide. 

 The cylinder is suspended in the jar by means of three stoat pieces 

 of brass or copper wire, which are softened in the flame ; each is 

 pushed through a hole, bent at one end as shown at c, and soldered 

 to the cylinder. One of the wires is to serve as a terminal, or for 

 making connection, in addition to holding the cylinder suspended ; 

 it should therefore be made longer than the others, as shown at a in 

 fig. 348 C. The zinc is amalgamated after the wires are fixed. 

 The copper is introduced as a circular disc of sheet copper, 2 or 3 cm 

 less in diameter than the mouth of the jar. If the bottom of the 

 jar is not flat, a cut is made into the disc of copper from the edge 

 to the centre, and the cut edges are made to overlap one another, so 

 that the whole forms a flat cone as shown in the figure. The 

 end of a copper wire, 6, is soldered to the middle of the copper disc, 

 and after washing away any residue of the soldering fluid, the wire 

 is heated, and the part of it which is inside the jar when the wire 

 stands perpendicular is covered all round with sealing-wax. The 

 portion not covered with sealing-wax is bent horizontal. The metal 

 pieces being placed in the jar, it is filled with solution of magnesic 

 sulphate, and a handful of crystals of cupric sulphate is thrown into 

 it, which dissolves and forms a deep blue saturated solution at the 

 bottom of the jar. When after the cell has been in use for some 



