354 



THE POPULAR EDUCATOR. 



the strength of the current. This voltameter, unlike the tan- 

 gent galvanometer, gives the actual and not merely the relative 

 strength, and is therefore very useful. 



Another way in which the decomposing effect of the current 

 may be seen is by means of a piece of paper moistened with a 

 solution of starch, to which a little iodide of potassium has 

 been added. As is well known, the presence of free iodine 

 produces a deep-blue colour in a solution of starch : when, how- 

 ever, the iodine is combined with any substance (as with potas- 

 sium in the present instance), no such effect is produced. 



Now place the battery wires on the surface of the paper, and 

 a distinct blue mark will be produced at the part where the 

 positive pole touches it. The explanation of this is that the 

 salt is decomposed into its two elements;. The potassium is 

 liberated at the negative pole, where, however, it is at once 

 converted into potash by combining with the oxygen of the 

 water, and may easily be detected by the use of appropriate 

 chemical tests. The iodine, on the other hand, is liberated at 

 the positive pole, and displays its presence by combining with 

 the starch and producing the characteristic blue colour. 



This principle was employed in the construction of Bain's 

 electro-chemical telegraph, which, although of no present value, 

 was most ingenious in construction. 



The decomposition of many substances can be easily shown 

 by a glass tube bent into the shape of the letter U, as shown 

 in Fig. 58. The tube is filled with a solution of the substance 

 to be decomposed, and the electrodes are placed one in each 

 limb. To exhibit the effects perfectly, the electrodes must be 

 made of some substance that is not acted upon by the consti- 

 tuents of the liquid. For most purposes platinum may be used ; 

 for some substances, however, electrodes of gas carbon are 

 necessary, as, for instance, in the decomposition of muriatic 

 acid, the chlorine of which would corrode the positive pole, if it 

 were composed of platinum. 



The decomposition of chemical salts can be well shown by this 

 arrangement. Let the tube, for instance, be filled with a solu- 

 tion of sulphate of potash (K 2 S0 4 ), coloured blue by a solution 

 of tincture of violets. As soon as the poles are immersed, the 

 liquid in the positive limb will begin to assume a red tint, while 

 that in the negative one will appear green. The salt has been 

 decomposed by the current, and its acid component liberated at 

 the positive pole, changing the blue colour to red. The base, 

 potash, has, on the other hand, been transferred to the negative 

 pole, and shown its presence there by turning the solution 

 green. A like effect would be produced if any other similar 

 salt were employed, the acid always being evolved at the posi- 

 tive pole and the base at the negative. 



In a future lesson we shall show what immense commercial 

 importance is attached to this power of electricity to decompose 

 metallic salts into their constituent elements, for upon it de- 

 pend the beautiful arts of electrotyping and electroplating. But 

 before proceeding to detail the methods pursued in these indus- 

 tries, it will be well to refer once more to the simple electrolysis 

 of water, for the purpose of reviewing the researches of different 

 experimenters, from whose labours the modern secondary bat- 

 tery has become a possibility. 



The discovery that the voltaic current would decompose 

 water into its constituent gases oxygen and hydrogen was 

 due to Nicholson and Carlisle, whose experiments led up to this 

 result within a year from the production of Volta's pile. 

 Shortly afterwards another curious result was noted by Gauthe- 

 rot, who found that the wires of platinum or silver, used to 

 decompose water, acquired the power to yield of themselves a 

 transient current after they had been detached from the pile or 

 cell used in the operation. To understand fully the nature of 

 this observation which we shall presently see is of great im- 

 portance let us suppose that by means of the apparatus shown 

 at Fig. 56 we have succeeded in producing the two gases in 

 their usual proportions. The battery cell has been detached, 

 and we fasten to the terminals in its place a good galvanometer. 

 We shall then see by the transient deflection of the needle that 

 a weak current is afforded by the apparatus. If we take the 

 trouble to attach the battery wires to the galvanometer, we 

 shall then note that what w"e may call the primary current is in 

 a different direction to the weak secondary current given by the 

 wires of the voltameter. This same curious reaction may be 

 observed not only in electrolytic cells, but in all battery cells. 

 This opposing current has, in fact, been quite a bugbear to 



electricians, and various ingenious methods have been devised 

 in different forms of batteries to obviate it. The phenomenon 

 is commonly known as polarisation. 



But very often in this world the difficulty of one forms the 

 opportunity for another. Whilst iae majority of electricians 

 were trying their best to stop this reactionary cuirent, which 

 interfered so much with the efficient working of their battery 

 cells, there were others who studied the phenomenon and endea- 

 voured to turn it to practical account. So long ago as 1803 

 only four years after the discovery of the voltaic pile Bitter, 

 of Jena, commenced an inquiry into the subject, and has left 

 behind him a record of many important experiments which he 

 subsequently carried out. The wires he used were of gold, and 

 the secondary current he obtained from them was sufficient to 

 throw the legs of a frog into violent contractions, as in the 

 famous experiment due to Galvani. Trying different kinds of 

 wires, he obtained the best result from platinum. Gold, silver, 

 copper, and bismuth follow next in order, according to their 

 efficiency ; but with zinc, tin, and lead, he obtained no results- 

 whatever. One experiment of Bitter's is worth noting. He 

 took a gold coin, and placing moistened cloth against its two 

 sides subjected it to the current from a powerful voltaic pile. 

 This coin afterwards yielded a secondary current, if kept in its 

 moist wrappings. But the gold wires he found would give up 

 their charge even if dried and put away for a time if they 

 were once more put into water. 



We cannot afford space to detail many similar experiments by 

 Volta, Davy, Faraday, and others, but must refer to the well- 

 known gas battery of Grove, which he produced in the year 

 1842. Looking once more at Fig. 56, let us imagine that 

 instead of only two inverted tubes for the reception of the dis- 

 engaged gases, we have a regular series arranged, and joined up 

 together like so many battery cells. We shall then form a, 

 correct notion of the appearance and arrangement of Grove's 

 gas battery, the oxygen tube of one couple being joined to the 

 hydrogen tube of the next, and so on through the series, each 

 tube having its little plate of platinum. To understand the 

 action of this gas battery, we must first of all note that the 

 metal platinum has a very curious property in the power of 

 absorbing upon its surface both hydrogen and oxygen gas, so 

 that we may take it for granted that when the two gases are 

 separated by means of the apparatus shown at Fig. 56, they not 

 only rise in bubbles so as to displace the acidulated water in 

 the tubes, but they also form films on the strips of platinum 

 foil. Now platinum in this state behaves towards ordinary 

 platinum very much in the same way that zinc behaves towards 

 copper in an ordinary battery cell. But the hydrogenised film 

 will show towards the oxygenised platinum a still stronger 

 electro-motive force, and it is the storage of the energy spent 

 in originally separating the two gases that the current in 

 Grove's gas battery is due. With fifty pairs, Grove produced 

 very powerful currents, sufficient indeed to support an electric 

 light. 



In 1860, M. Gaston Plante produced the secondary battery 

 cell shown at Fig. 59. This consists of two plates of lead 

 rolled together, but in reality separated by a sheet of cloth 

 rolled up with them. From each plate a strip of metal 

 proceeds through the cover of the cell for charging and dis- 

 charging purposes. The contained liquid is dilute sulphuric 

 acid. This kind of secondary battery was afterwards somewhat 

 modified by relinquishing the spiral form, and placing flat 

 plates of lead in a box-ljke tank, and connecting alternate 

 plates together ; but M. Plante afterwards went back to the 

 spiral form, as being on the whole more efficient and satis- 

 factory in every way. It must not be supposed that the Plante 

 battery is at once capable of affording a secondary current. 

 The cell requires a kind of education or, as its inventor ex- 

 presses it, it requires a certain time to " form " it. The lead 

 plates, so long as they remain clean, will not afford a current ; 

 indeed they are not in a fit state to give up what is sent in to 

 them from an external source. At first, when the current from 

 such a source, say from two or three Bunsen or Grove cells, is 

 applied to them, they behave very much like the pieces of 

 platinum foil contained in the voltameter shown at Fig. 56. 

 The two gases bubble from them, and but little attaches itself 

 as a film to the plates. But the plate by which the current 

 enters gradually changes its condition. It becomes covered by 

 a layer of peroxide of lead, which acts towards the other plate 



