Military Artillery Fortification and Siegecraft

MILITARY ; ARTILLERY ; FORTIFICATION AND SIEGECRAFT) . Also used for the "pitcher" and "catcher" in baseball; for a collection of utensils, primarily of hammered copper or brass, especially in the French term batterie de cuisine; and for the instruments of percussion in an orchestra.

Electric Battery.

This term was formerly applied to a col lection of Leyden jars, but is now used for a device for generating electricity by chemical action, or, more exactly, for a number of such devices joined up together. There are two main classes of electric batteries. In primary batteries, composed of a number of galvanic or voltaic "cells," "couples," or "elements," on the com pletion of the interactions between the substances on which the production of electricity depends the activity of the cells comes to an end, and can only be restored with the aid of a fresh supply of those substances ; in secondary batteries, also called storage batteries or accumulators (q.v.), the substances after the exhaus tion of the cells can be brought back to a condition in which they will again yield an electric current, by means of an electric cur rent 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 solu tion 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 ; the zinc in each cell being the negative and the copper the positive pole. During the time that the external connection is maintained between the two poles and the current passes in the wire, the zinc or positive plates are gradually dis solved and hydrogen gas is liberated at the surface of the copper or negative plates; but when the external connection is broken this action ceases. If the materials used in the cells were perfectly pure, probably the cessation would be complete. In practice, ever, even with open circuit a "local action" occurs 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 covery, applied by W. Sturgeon in 183o, 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 tromotive force which opposes the main current flowing from the zinc plate and diminishes its force. This phenomenon is known as "polarization," and various edies have been tried for the evils it duces in the practical use of primary teries. The plan usually adopted, is either to prevent the formation of the film or to introduce into the cell some izer" which will destroy it as it is formed by oxidizing the hydrogen to water (see also ELECTROLYSIS).

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 one 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.

Depolarization by Oxidation.

Sir W. R. Grove in 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 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 con venient 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 prepar ing the solution the sodium salt is preferable to the potassium, and chromic acid to either.

In the cell devised by Georges Leclanche in 1868 a solid depol arizer is employed, consisting 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 electrolyte is a solu tion 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.

Recent Cells.

In the Darimont cell, fig. i (cf. Engineer, 1924, 137, 636), which is of the two-fluid type separated by a porous pot, the plates consist of carbon and zinc. The liquid depolarizer around the carbon consists largely of ferric chloride ; whilst the electrolyte around the zinc consists of an emulsion formed of sodium chloride, calcium carbonate and other materials, so that the electrolyte is always maintained neutral and local action is consequently negligible. As the internal resistance remains prac tically constant when a given voltage has to be maintained, the number of cells required is less than usual and the voltage re mains very steady during the discharge. The capacity of the cell is high and it is very suitable for use with valves for wireless work in place of accumulators. Results of tests made on a cell of this type of a normal capacity of 5o amp. hrs. are shown in fig. 2.

The cell was discharged through a resistance of i o ohms down to 6 volts. The total current yielded amounted to 49 amp. hrs. or 62.1 watt hours. The zinc consumed per amp. hr. was 1.25 grm., the theoretical figure being 1.213.

The Fery primary battery (Journ. Sci. Instr. 1926, 3, is a modification of the Leclanchc cell. The corrosion of the zinc at the surface of the liquid and the clogging of the porous pot with insoluble crystals which occurs in the Leclanche cell are obviated by removing the zinc from the surface and placing it as a flat plate in the bottom of the cell and by dispensing with the porous pot and the contained manganese dioxide. The carbon is in the form of a long cylinder depending from the top of the cell so that its base is a short distance above the zinc plate, and a solution of sal ammoniac forms the electrolyte. When the battery is in action the zinc dissolves according to the reaction:— = and the hydrogen rises up and is deposited on the carbon and es capes, while the ammoniacal solution, being lighter, also rises to the surface, leaving the heavier zinc chloride solution at the bot tom. For low currents the ammonia diffuses into the air, but with heavy currents, when it is produced too fast to escape, it unites with the zinc chloride to form a hydrate as follows:— Zn (OH The hydrated oxide is, however, very soluble both in the zinc chloride solution and in the ammonium hydrate, so that it is only in the neutral zone between the two solutions or at about a quar ter or a third of the height of the carbon that the crystals of the double oxychloride of zinc and ammonium are deposited, and as this is well away from the base of the carbon cylinder near to the zinc, the action of the cell is but little interfered with. The hydrogen deposited on the carbon, which would ordinarily cause polarization is oxidized through absorption of atmospheric oxygen by the carbon in the portion above the electrolyte. The carbon thus acts as a catalyst in effecting the oxidation of the hydrogen by the air. Fig. 3 shows sections of three types of these cells.

In tests made during constant discharge the internal resistance of the battery was about 0.25 ohm per cell three months after the commencement, rising to about i ohm per cell after nearly two years, and the zinc plates dissolved quite uniformly. Cells io cm. square and 22 cm. high with a zinc plate weighing 16o grm. are claimed to yield a total energy, either in continuous or interrupted service, amounting to 125 amp. hours.

In the "A.D." Primary Cell (cf. Elect. Rev. [London], 1924, 94, 433) made by Le Carbone Co. the central carbon plate to which the connecting terminal is fixed is held between extremely porous carbons of special manufacture. The surfaces of the car bons are so prepared that they allow gases to pass through, while they remain impervious to liquids. The carbon blocks are sur rounded by wrappings of cloth bound in turn with string and rub ber bands. The upper surfaces of the carbon are left well out of the electrolyte and allow depolarization to proceed through con tact with air. Owing to the absence of any chemical depolarizing agent, local actions are eliminated and deterioration on standing thereby avoided. The voltage remains constant during discharge, and the ampere-hour capacity is stated to be higher than that of any other primary cell of equal dimensions.

Dry Cells.

The term "dry" cell is, in this application, used only in a relative sense to denote that the contents of the cell are in a pasty form and not sufficiently fluid to flow if the cell is in verted. The first practical primary cell of the type in use at pres ent was produced by Gassner in 1888. This cell consisted of a zinc can serving as anode and also as the container for the cell, a carbon rod surrounded by the depolarizing mixture, which was wrapped in cloth, and the electrolyte in the form of a jelly. The open-circuit voltage of this cell was about 1.3 volts, and its short circuit current about 6 amperes. At first these were made of a size 21 in. X 6 in. known as the standard dry cell. There soon appeared a miniature type known as the "flashlight" cell. About 1904 the demand for dry cells increased immensely owing to their use in automobile ignition sys tems, though the starting and lighting systems at present in use finally put an end to that demand. A further extensive application for these cells which still contin ues is for the operation of tele phones, for which a special type of cell has been developed. Dur ing the last few years a large de mand for dry cells has resulted from their application to the "A" circuits of radio receiving sets, and this in turn has given rise to a special type of cell adapted to the heavy demands imposed upon it in radio service. The application of batteries of miniature cells of this type for operating the "B" circuit also caused a very large expansion in the industry.

In the construction most commonly employed at present the cylindrical container consists of zinc, which serves as the zinc plate. Formerly a layer of paste made of flour, plaster of Paris, ammonium chloride, and zinc chloride was then coated around the inside of the zinc container and allowed to set. In a procedure at present largely followed a double layer of paper board is used in place of the separating paste. A carbon rod is then mounted in the centre of the case and packed around with a depolarizing paste the exact composition of which is held as a trade secret, but is stated to be approximately as follows:— Manganese peroxide . . zo Carbon or graphite . . . . . . . . . . io Sal ammoniac . 2 Zinc chloride . . . Sufficient water is added to make this mixture into a paste, when the zinc chloride prevents the contents becoming too dry.

For the general construction of typical dry cells see fig. 5.

Sometimes the electrolyte is made into a jelly with such col loidal material as gum tragacanth, agar agar, gelatin, flour or starch. The electrolyte is there fore unspillable, whether the cell is completely sealed over the top, as is most common in American practice, or is provided with a vent for the escape of gas, as is common in European practice. As the cell is discharged the manganese dioxide is reduced and the effective surface of the elec trode travels toward the carbon rod, which is in the centre axially with the cell. This carbon rod only serves to conduct the current out of the mixture to the terminal.

In the common bag type of dry cell battery the manganese di oxide mixture is contained in a cloth bag or sack. The carbon rod with its surrounding mixture is wrapped in muslin and tied with string, forming a unit which can be placed in the zinc can, leav ing sufficient space between the two for the electrolyte in the form of a paste. Spacers to separate the bag from the zinc can are desirable, but are not always used. These are commonly rub ber bands in the small cells, such as are used for flashlight bat teries, or Manila cord, which is of considerable size in some of the foreign makes of cells. The solution of sal ammoniac and zinc chloride is thickened with flour or other similar materials and may also contain other ingredients, differing with manufacturers and kept secret by them.

In the Siemens dry cell (Electrical Review [London] , 1927, roc., 3), which is constructed for radio purposes, the carbon electrodes are made from purified retort carbon, which is ag glomerated by pressure and baking, and the second electrode is made of sheet zinc and forms the outside canister. The depolariz ing paste which is tamped in the vessel consists of manganese peroxide and carbon together with some graphite, to reduce the internal resistance, and gum. The space between the zinc canister and the depolarizer is filled by pouring in a thin exciting paste consisting of a solution of sal ammoniac, flour and plaster of Paris ; the cell is finally filled up with seed husks which allow for