650 CONDENSATION OF GALVANIC ELECTRICITY. 



it should not stop near the edge and then be moved across the edge 

 to the back of the plate, as in the first operation, but it should in one 

 continuous movement proceed from the middle, across the edge, and 

 along the back, to where the tinfoil ends. This must of course be 

 done repeatedly and in every possible direction, until no more paste 

 can be squeezed out. Afterwards a round disc of tinfoil is pasted 

 upon the back of each plate ; it is about l cm less in diameter than 

 the plate, and has in the middle a hole about 3 cm wide, which allows 

 of its being passed over the handle. 



The insulating layer should be as thin as possible. The best way 

 is to varnish the free surface of each plate with the clear portion of 

 shellac solution, as has previously been described for glass (see 

 page 561) ; but extreme caution is required to heat the plates only 

 just sufficiently to dry the varnish without rendering it turbid and 

 dull ; the slightest over-heating blisters the tinfoil, renders the sur- 

 face uneven, and therefore the whole useless. A thin uniform coat 

 of solution of sealing-wax (see page 561) is more easily produced 

 than the layer of shellac varnish, but it is inferior for the present 

 purpose. One or two coatings, laid on at intervals of about twenty- 

 four hours, will be required to render the insulating layer uniform. 



The edges of each plate should also be varnished, but not the 

 back. The stick of sealing-wax which carries the lower plate is last 

 of all fixed to a wooden foot. 



It has already been stated that a piece of zinc be- 

 comes negatively charged, and a piece of copper posi- 

 tively, if they are immersed together in a conducting 

 liquid. The same happens if the two pieces of metal 

 are not immersed in the liquid, but placed otherwise in 

 contact with it. The most simple mode of producing 

 not only contact of the metals with the liquid, but also 

 of connecting them at the same time with the con- 

 denser, is that represented in fig. 335. Two strips, 

 one of copper, the other of jginc, k and z in the figure, 

 each 6 om long and l om wide, and bent as in the figure, 

 aye provided with handles of sealing-wax; at the ends 

 which are bent they touch the condenser, and at the 

 Other en<Js which are straight they hold between them 



