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 a fresh supply of those substances; in secondary batteries, also called storage batteries or s (q.v.), the substances after the exhaustion of the cells can be brought back to a condition in which they will again yield an electric current, by means of an electric current passed through them in the reverse direction. The first primary battery was constructed about 1799 by Alessandro Volta. In one form, the “voltaic pile,” he placed a series of pairs of copper and zinc disks one above the other, separating each pair from the one above it by a piece of cloth moistened with a solution of common salt. In another form, the “couronne de tasses,” he took a number of vessels or cells containing brine or dilute acid, and placed in each a zinc plate and a copper plate; these plates were not allowed to touch each other within the vessels, but each zinc plate was connected to the copper plate of the adjoining vessel. In both these arrangements an electric current passes through a wire which is connected to the terminal plates at the two ends of the series. The direction of this current is from copper to zinc; within each cell itself it is from zinc to copper. The plate to which the current flows within the cell is the negative plate, and that from which it flows the positive plate; but the point on the negative plate at which the current enters the external wire is the positive pole, and the point on the positive plate at which it leaves the external circuit the negative pole. During the time that the external connexion is maintained between the two poles and the current passes in the wire, the zinc or positive plates are gradually dissolved, and hydrogen gas is liberated at the surface of the copper or negative plates; but when the external connexion is broken this action ceases. If the materials used in the cells were perfectly pure, probably the cessation would be complete. In practice, however, only impure commercial zinc is available, and with this corrosion continues to some extent, even though the external circuit is not closed, thus entailing waste of material. This “local action” is explained as due to the fact that the impurities in the zinc plate form miniature voltaic couples with the zinc itself, thus causing its corrosion by voltaic action; and an early improvement in the voltaic cell was the discovery, applied by W. Sturgeon in 1830, that the evil was greatly reduced if the surface of the zinc plates was amalgamated, by being rubbed with mercury under dilute sulphuric acid. Another disadvantage of the simple cell composed of copper and zinc in dilute acid is that the current it yields rapidly falls off. The hydrogen formed by the operation of the cell does not all escape, but some adheres as a film to the negative plate, and the result is the establishment of a counter or reverse electromotive force which opposes the main current flowing from the zinc plate and diminishes its force. This phenomenon is known as “polarization,” and various remedies have been tried for the evils it introduces in the practical use of primary batteries. Alfred Smee in 1839 modified the simple copper-zinc couple excited by dilute sulphuric acid by substituting for the copper thin leaves of platinum or platinized silver, whereby the elimination of the hydrogen is facilitated; and attempts have also been made to keep the plates free from the gas by mechanical agitation. The plan usually adopted, however, is either to prevent the formation of the film, or to introduce into the cell some “depolarizer” which will destroy it as it is formed by oxidizing the hydrogen to water (see also ).

The former method is exemplified in the cell invented by J. F. Daniell in 1836. Here the zinc stands in dilute sulphuric acid (or in a solution of zinc sulphate), and the copper in a saturated solution of copper sulphate, the two liquids being separated by a porous partition. The hydrogen formed by the action of the cell replaces copper in the copper sulphate, and the displaced copper, instead of the hydrogen, being deposited on the copper plate polarization is avoided. The electromotive force is about one volt. This cell has been constructed in a variety of forms to suit different purposes. In a portable form, designed by Lord Kelvin in 1858, the copper plate, soldered to a gutta-percha covered wire, is placed at the bottom of a glass vessel and covered with crystals of copper sulphate; over these wet sawdust is sprinkled, and then mere sawdust, moistened with solution of zinc sulphate, upon which is placed the zinc plate. The Minotto cell is similar, except that sand is substituted for sawdust. In these batteries the sawdust or sand takes the place of the porous diaphragm. In another class of batteries the diaphragm is dispensed with altogether, and the action of gravity alone is relied upon to retard the interdiffusion of the liquids. The cell of J. H. Meidinger, invented in 1859, may be taken as a type of this class. The zinc is formed into a ring which fits the upper part of a glass beaker filled with zinc sulphate solution. At the bottom of the beaker is placed a smaller beaker, in which stands a ring of copper with an insulated connecting wire. The mouth of the beaker is closed by a lid with a hole in the centre, through which passes the long tapering neck of a glass balloon filled with crystals of copper sulphate; the narrow end of this neck dips into the smaller beaker, the copper sulphate slowly runs out, and being specifically heavier than the zinc sulphate it collects at the bottom about the copper ring. In Lord Kelvin’s tray-cell a large wooden tray is lined with lead, and is covered at the bottom with copper by electrotyping. The zinc plate is enveloped in a piece of parchment paper bent into a tray shape, the whole resting on little pieces of wood placed on the bottom of the leaden tray. Copper sulphate is fed in at the edge of the tray and zinc sulphate is poured upon the parchment. A battery is formed by arranging the trays in a stack one above the other.

Various combinations have been devised in which the hydrogen is got rid of more or less completely by oxidation. Sir W. R. Grove in 1839 employed nitric acid as the oxidizing agent, his cell consisting of a zinc positive plate in dilute sulphuric acid, separated by a porous diaphragm of unglazed earthenware from a platinum negative immersed in concentrated nitric acid. Its electromotive force is nearly two volts, but it has the objection of giving off disagreeable nitrous fumes. R. W. von Bunsen modified Grove’s cell by replacing the platinum with the much cheaper material, gas carbon. Chromic acid is much used as a depolarizer, and cells in which it is employed are about as powerful as, and more convenient than, either of the preceding. In its two-fluid form the chromic acid cell consists of a porous pot containing amalgamated zinc in dilute sulphuric acid, and a carbon plate surrounded with sulphuric acid and a solution of potassium or sodium bichromate or of chromic acid. But it is commonly used in a one-fluid form, the porous pot being dispensed with, and both zinc and carbon immersed in the chromic acid solution. Since the zinc is dissolved even when the circuit is not closed, arrangements are frequently provided by which either the zinc plate alone or both plates can be lifted out of the solution when the cell is not in use. In preparing the solution the sodium salt is preferable to the potassium, and chromic acid to either. In the cell devised by Georges Leclanché in 1868 a solid depolarizer is employed, in the shape of manganese dioxide packed with fragments of carbon into a porous pot round a carbon plate. A zinc rod constitutes the positive plate, and the exciting fluid is a solution of sal-ammoniac. Sometimes no porous pot is employed, and the manganese dioxide and granulated carbon are agglomerated into a solid block round the carbon plate. The electromotive force is about one and a half volt. The cell is widely used for such purposes as ringing electric bells, where current is required intermittently, and for such service it will remain effective for months or years, only needing water to be added to the outer jar occasionally to replace loss by evaporation. On a closed circuit the current rapidly falls off, because the manganese dioxide is unable to oxidize all the hydrogen formed, but the cell quickly recovers after polarization. The so-called “dry cells,” which came into considerable use towards the end of the 19th century, are essentially Leclanché cells in which the solution is present, not as a liquid, but as a paste formed with some absorbent material or gelatinized. Black oxide of copper is another solid depolarizer, employed in the Lalande cell. In the Edison-Lalande form the copper oxide is suspended in a light copper frame. The exciting solution consists of one part of caustic soda dissolved in three parts by weight of water, and to prevent it from being acted on by the