GALVANISM. 



resistance. Now, if this resistance be small, the 

 single cell will transmit the polarity as easily as 

 the battery. That is to say, with a short good 

 conductor between the poles, a single cell will give 

 as strong- a current, and deflect the magnetic 

 needle as much, as a hundred. The superior 

 electro-motive force of the battery has, as it were, 

 no opportunity of exhibiting its power. With a 

 long and difficult conductor between the poles, it 

 is not the same. Here the strength of the bat- 

 tery is seen. One cell may nearly or even utterly 

 fail to establish a polarity throughout such a 

 circuit ; yet the battery will push the polarity on 

 in the face of it, and give a strong current, while 

 the cell gives little or none at alL 



Requisites of a good Battery. The objects to be 

 attained in a battery are chiefly two : first, that 

 the strength of the battery, and, therefore, the 

 electro-motive power of each cell, should be as 

 great as possible ; and second, that the battery 

 should be able to keep up steadily, and for a long 

 time, its original strength. The first condition 

 must be provided for by selecting metals widely 

 apart in the electro-chemical list, and by avoiding 

 local action. As to the second condition, it is not 

 such an easy matter to secure a battery uniform 

 in its action. Several causes operate to impair 

 its strength in course of time. The acid gets weak, 

 and must be strengthened from time to time. An- 

 other injurious influence is at work always when 

 the plates are placed in one liquid, and face each 

 other directly. This is a tendency of particles of 

 the one plate to pass to the other, and so to reduce 

 that difference in the surfaces which is essential 

 to the action. In the copper-zinc couple, for 

 example, the dissolved sulphate of zinc forms a 

 deposit on the copper plate, while some of the 

 copper also passes to the zinc ; so that in course 

 of time the two surfaces become almost alike, and 

 the current almost nothing. This is a grievous 

 fault, but, as we shall see, it can be prevented by 

 placing a porous partition between the plates. It 

 gets soaked with the liquid, and so allows com- 

 munication to pass. Yet it is close enough to 

 prevent the exchange of metallic particles. 



Besides these, there is another very serious 

 cause of weakening, which requires special pro- 

 vision. It is known as polarisation of the plates. 

 In all batteries, where hydrogen is set free at the 

 negative plate, there is a tendency of the hydrogen 

 bubbles to adhere to the plate, and a rapid fajl in 

 strength of current is the result The hydrogen 

 not only forms a non-conducting layer over the 

 surface of the plate, but, what is worse, gives rise 

 to a counter or a secondary current. This 

 hydrogen deposit is prevented by using two 

 liquids in place of one. Both must be conducting, 

 and the unaffected or negative element is im- 

 mersed in a separate liquid, of such a nature that 

 the liberated hydrogen can act chemically on it, 

 and thus be absorbed as fast as it is formed. 



Various Forms of Cells and Batteries. (i.) 

 CruikshanKs Trough Battery was the first im- 

 provement on Volta's pile, of which it is but 

 another form. Pairs of copper and zinc plates, 

 soldered together, are fixed in a wooden trough, 

 coated with some insulating varnish. The cells 

 are filled with dilute acid, and correspond exactly 

 to Volta's cloth discs. The defects of assimilation 

 and of polarisation of the plates, described above, 

 combine to make this battery unsatisfactory. 



(2.) Wollaston's Battery is shewn in fig. 20, and 

 is an improvement on the last The copper plate 

 is bent up so as to face the zinc on each side. 



Fig. 20. 



This clearly brings twice as much zinc surface into 

 action, and so doubles the quantity of electricity 

 which each cell will give. Each couple is im- 

 mersed in a separate trough, to insure insulation. 

 The zinc of one cell is connected to the copper of 

 the next by a strip of copper soldered to each ; 

 and the whole may be raised out of the cells, 

 when not in action, by means of the wooden rod 

 supporting them. 



(3.) Smee's Battery is, in general, similar to the 

 former ; only, in place of a copper, there is a thin 

 stiver plate, which is more electro-negative, and 

 gives, therefore, greater current power. The posi- 

 tions of the plates are also reversed, to save silver, 

 there being a zinc on each side of the silver, 

 and kept from touching it by strips of wood or 

 cork. The zinc plates are bound together by a 

 coupling, to which the connection from the next 

 silver is fixed. Polarisation of plates may be 

 lessened by covering the surface of the silver with 

 finely-divided platinum. 



(4.) Danielfs Battery, invented in 1826, was one 

 of the first, and is still one of the best arrange- 

 ments for constancy. It is a zinc and copper 

 combination, the copper being in the shape of a 

 jar, and serving as the outer dish of the cell. 



Fig. 21 represents a DanielFs cell The zinc, se, 

 is formed into a rod, and is placed inside a porous 

 jar, d, of unglazed porcelain or 

 earthenware, which again stands 

 inside the copper cylinder, c. In 

 the porous dish, dilute sulphuric 

 acid (one part acid to 10 or 12 

 of water) serves to excite the zinc. 

 As a conducting and absorbent 

 liquid, between the porous vessel 

 and the copper, is put a strong 

 solution of the blue crystals known 

 as sulphate of copper or blue vitriol. 

 Let us have a clear idea of the 

 action of this sulphate of copper 

 solution. It is a beautiful arrange- 

 ment. Sulphate of copper is what 

 is formed when sulphuric acid acts 

 on copper ; and chemistry informs 

 us that, in the process, an atom of 

 copper expels and takes the place 

 of the atom of hydrogen which is present in each 



Fig. 21. 



