The great fault is rather a misdirection than a lack of energy, which often seems to be due to an entire ignorance of, or failure to grasp, fundamental principles, and consequently both time and money, that might have produced some good results if expended in the right direction, are wasted in attempting to accomplish the impossible; and, although he naturally fails in the endeavour, the inventor often manages to persuade both himself and other people that he has actually achieved success; in fact, those who are suffering from what has been aptly termed the "primary-battery disease," can only be classed with such paradoxists as the flat-earth theorists, the substantial philosophers, and the perpetual motion-ists.
To make any substantial progress one must give up ringing the changes on sulphuric versus hydrochloric acid, bichromate of potash versus chromic acid, nitrate of soda versus nitric acid, and go and strike out on more original lines, but always remembering that all battery phenomena take place- in strict accordance with the laws of chemical combination and the conservation of energy. Some of these may briefly be put as follows: - 1. Whenever two or more substances combine chemically, they liberate a certain amount of energy, which is called the energy of chemical combination. 2. The amount of energy liberated depends on the chemical affinity between them. If they have a strong tendency to combine, it will be large; but if they only unite with difficulty, and under the most favourable circumstances, it will be small. 3. To break up a chemical compound into its constituent substances or "elements,*' will require the expenditure of exactly as much energy as was liberated by those elements when combining, so that if a substance leaves a compound of which it forms part in order to combine with some other substance for which it has a greater affinity, the resultant amount of energy set free will be the difference, between these two actions. 4. The greater portion of the energy freed by chemical combination usually takes the form of heat, and raises the temperature of the bodies concerned to a greater or less degree.
The familiar phenomena of "burning" or "combustion " offer us the most important example of the above laws that can be selected; for, besides being the method universally employed for obtaining both heat and light, it is at present almost the only known means of producing mechanical energy artificially and on a large scale. Here such substances as wood and coal, which are compounds consisting principally of hydrogen and carbon (" hydrocarbons "), combine with the oxygen of the air to form carbon dioxide (C02) and water (H20). The chemical affinity of carbon and hydrogen for each other is small, while both have a strong affinity for oxygen; consequently but little energy is absorbed in breaking up the original compound, and there is a tremendous generation of heat which maintains the burning substances at a temperature sufficient for the emission of both radiant heat and light in large quantities, although the main portion of the energy passes off in the hot gases. " Rusting " or " oxidation" is another well-known example of the same nature, but metals take the place of the hydrogen and carbon, and the action is ordinarily so extremely slow that the heat generated in the process is dissipated into the air and neighbouring bodies long before it can produce any perceptible rise of temperature, and there is*nothing therefore to suggest "combustion" in the ordinary sense of the word.
As already implied, a galvanic or voltaic cell (also termed "battery," although, strictly speaking, this word should only be used in reference to a number of individual cells) is essentially an apparatus for the direct conversion of the energy of chemical combination into electrical energy instead of into heat. In almost all*batteries at present of any practical value, the electrical energy is obtained by the combination of zinc with sulphuric acid to form sulphate of zinc, or, as it may be termed, the "combustion " of zinc in sulphuric acid.
Every grain of zinc converted into zinc sulphate in this way sets free a fixed and definite amount of energy, a part of which is absorbed in other chemical changes taking place at the same time, while under ordinary circumstances the remainder appears as heat, and indicates its presence by raising the temperature of the liquid. If, however, a piece of copper is placed in the acid with the zinc, and the two are connected by a wire outside the liquid, the latter portion of the energy shows itself as a current of electricity passing through the acid from the zinc to the copper, and back again by the connecting-wire; but the point which we wish to emphasise is that, no matter in what form the energy may be present, whether it remains as chemical energy, whether it is directly converted into heat in the liquid, whether it appears as electricity (being afterwards reduced to heat partly in the cell, and partly in the outside wire), or whether, as is most usual, it s divided in a varying proportion between all the three, the total amount available is in each case precisely the same, and it follows, therefore, that the quantity of electricity to be obtained by the consumption of a given weight of zinc is strictly limited, the maximum output being reached when the whole of the energy takes this form.
As a matter of fact, no cell absolutely realises this condition, although certain forms (not of much practical use) do so theoretically; but in the greater number the energy utilised electrically.does not, and cannot, exceed 40 to 90 per cent, of the total amount; and it is important to remember that the electromotive force of a cell, when freshly set up, affords a direct indication of the utmost percentage that could be so utilised, provided there were no losses due to "local action " and " polarisation."
It has been determined from theoretical considerations that the electromotive force of zinc combining with S04 to form ZnS04 is 2.36 volts, and the nearer the E.M.F. of any given cell to this value the greater is its possible efficiency - in other words, the maximum energy that can be utilised electrically: the total energy liberated by the zinc:: the E.M.F. of the cell: 2.36. For a Daniell cell this ratio will be 1.08 / 2.36 or 45 per cent.; for a Bunsen cell 1.8/2.36, for 75 per cent.; for a bichromate cell 2/2.36, or 85 per cent., and so on. To account for the remainder of the energy we must look a little closer into the chemical changes taking place in the cell.