pasty amalgam. The iron particles do not dissolve in the mercury, but float up to the surface, whence the hydrogen bubbles which may form speedily carry them off. As the zinc in this pasty amalgam dissolves into the acid the film of mercury unites with fresh portions of zinc, and so presents always a clean bright surface to the liquid. A newer and better process is to add about 4 per cent of mercury to the molten zinc before casting into plates or rods. If the zinc plates of a battery are well amalgamated there should be no evolution of hydrogen bubbles when the circuit is open. Nevertheless there is still always a little wasteful local action during the action of the battery. Jacobi found that while one part of hydrogen was evolved at the positive pole, 336 parts of zinc were dissolved at the negative pole, instead of the 32.5 parts which are the chemical equivalent of the hydrogen. 163. Polarisation.-The bubbles of hydrogen gas liberated at the surface of the copper plate stick to it in great numbers, and form a film over its surface; hence the effective amount of surface of the copper plate is very seriously reduced in a short time. When a simple cell, or battery of such cells, is set to produce a current, it is found that the strength of the current after a few minutes, or even seconds, falls off very greatly, and may even be almost stopped. This immediate falling off in the strength of the current, which can be observed with any galvanometer and a pair of zinc and copper plates dipping into acid, is almost entirely due to the film of hydrogen bubbles sticking to the copper pole. A battery which is in this condition is said to be "polarised." 164. Effects of polarisation.—The film of hydrogen bubbles affects the strength of the current of the cell in two ways. Firstly, It weakens the current by the increased resistance which it offers to the flow, for bubbles of gas are bad conductors; and, Secondly, It weakens the current by setting up an opposing electromotive-force; for hydrogen is almost as oxidisable a substance as zinc, especially when freshly deposited (or in a "nascent " state), and is electropositive, standing high in the series on p. 69. Hence the hydrogen itself produces a difference of potential, which would tend to start a current in the opposite direction to the true zinc-to-copper current. It is therefore a very important matter to abolish this polarisation, otherwise the currents furnished by batteries would not be constant. 165. Remedies against Internal Polarisation. -Various remedies have been practised to reduce or prevent the polarisation of cells. These may be classed as mechanical, chemical, and electro-chemical. 1. Mechanical Means.—If the hydrogen bubbles be simply brushed away from the surface of the positive pole, the resistance they caused will be diminished. If air be blown into the acid solution through a tube, or if the liquid be agitated or kept in constant circulation by siphons, the resistance is also diminished. If the surface be rough or covered with points, the bubbles collect more freely at the points and are quickly carried up to the surface, and so got rid of. This remedy was applied in Smee's Cell, which consisted of a zinc and a platinised silver plate dipping into dilute sulphuric acid; the silver plate, having its surface thus covered with a rough coating of finely divided platinum, gave up the hydrogen bubbles freely; nevertheless, in a battery of Smee Cells the current falls off greatly after a few minutes. 2. Chemical Means.—If a highly-oxidising substance be added to the acid it will destroy the hydrogen bubbles whilst they are still in the nascent state, and thus will prevent both the increased internal resistance and the opposing electromotive - force. Such substances are bichromate of potash, nitric acid, and bleaching powder (so-called chloride of lime). These substances, however, would attack the copper in a zinc-copper cell. Hence they can only be made use of in zinc-carbon or zinc platinum cells. circuit is open. Nitric acid also attacks zinc when the Hence it cannot be employed in the same single cell with the zinc plate. In the Bichromate Battery, invented by Poggendorf, bichromate of potash is added. to the sulphuric acid. This cell is most conveniently made up as a "bottle battery " (Fig. 72), in which a plate of zinc is the -pole, and a pair of carbon plates, one on each side of the zinc, are joined together at the top as a + pole. As this solution acts on the metal zinc when the circuit is open, the zinc plate is fixed to a rod by which it can be drawn up out of the solution when the cell is not being worked. Other cases of chemical prevention of polarisation are mentioned in describing other forms of battery. Fig. 72. 3. Electro-chemical Means.-It is possible by employing double cells, as explained in the next Lesson, to so arrange matters that some solid metal, such as copper, shall be liberated instead of hydrogen bubbles, at the point where the current leaves the liquid. This electrochemical exchange entirely obviates polarisation. 166. Simple Laws of Chemical Action in the Cell. We will conclude this section by enumerating the two simple laws of chemical action in the cell. I. The amount of chemical action in the cell is propor tional, to the quantity of electricity that passes through it, -that is to say, is proportional to the strength of the current while it passes. One coulomb1 of electricity in passing through the cell liberates (or 000010352) of a gramme of hydrogen, and causes (or 00033644) of a gramme of 32.5 96600 zinc to dissolve in the acid. II. The amount of chemical action is equal in each cell of a battery consisting of cells joined in series. The first of these laws was thought by Faraday, who discovered it, to disprove Volta's contact theory. He foresaw that the principle of the conservation of energy would preclude a mere contact force from furnishing a continuous supply of current, and hence ascribed the current to the chemical actions which were proportional in quantity to it. How the views of Volta and Faraday are to be harmonised has been indicated in the last paragraph of Art. 72. LESSON XV.-Voltaic Ratteries. 167. A good Voltaic Battery should fulfil all or most of the following conditions : I. Its electromotive-force should be high and con stant. 2. Its internal resistance should be small. 3. It should give a constant current, and therefore must be free from polarisation, and not liable to rapid exhaustion, requiring frequent renewal of the acid. 4. It should be perfectly quiescent when the circuit is open. 5. It should be cheap and of durable materials. 1 For the definition of the coulomb, or practical unit of quantity of electricity, see Art. 323. 168. No single battery fulfils all these conditions, however, and some batteries are better for one purpose and some for another. Thus, for telegraphing through a long line of wire a considerable internal resistance in the battery is no great disadvantage; while, for producing an electric light, much internal resistance is absolutely fatal. The electromotive-force of a battery depends on the materials of the cell, and on the number of cells linked together, and a high E.M.F. can therefore be gained by choosing the right substances and by taking a large number of cells. The resistance within the cell can be diminished by increasing the size of the plates, by bringing them near together, so that the thickness of the liquid between them may be as small as possible, and by choosing liquids that are good conductors. Of the innumerable forms of battery that have been invented, only those of first importance can be described. Batteries may be classified into two groups, according as they contain one or two fluids, or electrolytes. SINGLE-FLUID CELLS. 169. The simple cell of Volta, with its zinc and copper plates, has been already described. Cruickshank suggested to place the plates vertically in a trough, producing a more powerful combination. Dr. Wollaston proposed to use a plate of copper of double size, bent round so as to approach the zinc on both sides, thus diminishing the resistance. Smee, as we have seen, replaced the copper plate by platinised silver, and Walker suggested the use of plates of hard carbon instead of copper or silver, thereby saving cost, and at the same time increasing the electromotive - force. The simple bichromate cell (Fig. 72) is almost the only single-fluid cell free from polarisation, and even in this form the strength of the current falls off after a few minutes' working, owing to the chemical reduction of the liquid. Pabst uses an iron-carbon cell with perchloride of iron as the exciting liquid. The iron dissolves and chlorine is at first evolved; but without polarisation; the liquid regenerating itself |