cylinder, as illustrated in Figs. 204 and 204a, which show respectively the complete "Aylmer-Leclanché" cell and a section of its interior.

The curves in Fig. 205 give the results of tests made on three types of Leclanché cell, when the outside resistance in each case was maintained constant at 10 ohms, the plan adopted by the Post Office for testing cells, and

Fig. 205.-Curves showing the Time-fall of current with an external resistance of 10 ohms in each case.

we see that the current under these circumstances fell to half its value in six, seventeen, and forty-one days respectively with the ordinary porous pot Leclanché cell, the Leclanché-Barbier cell, and the Aylmer-Leclanché cell. It will be observed that in each case the polarisation is somewhat rapid at the beginning, and especially rapid in the case of the ordinary porous pot Leclanché cell.

Example 131.-If 2 lbs. of zinc have been consumed in a Leclanché battery, how much sal ammoniac has been utilised in the same time?

Answer.--About 3.3 lbs.

Example 132.-Compare the rates of using up manganese peroxide and sal ammoniac in a Leclanché cell. Answer. Approximately, as 163 to 100.

Example 133.-What is the cost of the material consumed in 6 Leclanché cells in series when developing a current of 0.1 ampere for three hours a day for 200 days, if 10 per cent. of the material used is wasted through local action? Take the price of zinc as ląd. per lb., of sal ammoniac 45s. per cwt., and of manganese peroxide as 14s. per cwt.

Answer. Cost of zinc, about 2d.

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sal ammoniac, about 81d. manganese peroxide, about 41d. Example 134.-With the prices of materials given in the preceding question, what would be the expense of supplying a Board of Trade unit to an external circuit by means of a battery of 6 Leclanché cells whose resistance was equal to that of the external circuit, and the mean value of whose E.M.F. during the time was 1.2 volt per cell?

Answer.-Cost of zinc, about 8d.

sal ammoniac, about 2s. 10d. manganese peroxide, about 1s. 5d. Hence, disregarding waste due to local action, the cost of supplying the Board of Trade unit would be about 5s. Example 135.-If the resistance external to a Leclanché battery be very large compared with that of the battery itself, and if the average E.M.F. of each cell may be taken as 1.5 volt, what is the cost of developing a Board of Trade unit, with the prices of materials given in example 133?

Answer.-2s.

140. Dry Cells. Many attempts have been made to construct a cell which could be turned upside down or used in any position without interfering with its action.* Volta constructed a battery of zinc and copper plates

* "Primary Batteries in Theory and Practice," W. R. Cooper, Electrician, vol. xxxi., 1893.

P*

with pieces of moist cloth inserted between them. Zamboni used discs of paper covered on one side with tin and on the other with manganese peroxide; but batteries of this type, although they could produce a large E.M.F. when a sufficiently large number of elements was employed, were only able to furnish an extremely small current in consequence of their large internal resistance. Wolf, Keisen, and Schmidt tried to make a "dry cell" of moderate resistance by mixing sawdust with cellulose. Desruelles filled a Leclanché cell with asbestos fibre and spun glass; Pollak employed a gelatine glycerine; but the first to construct a dry cell which could be successfully used to produce an appreciable current was Gassner in 1888.

The "Gassner's" dry cell was a form of Leclanché cell, the plates being formed of carbon and zinc, the latter being made in the shape of a pot to contain the jelly which surrounded the carbon rod. This jelly was composed of sal ammoniac, zinc chloride and oxide, calcium sulphate, and water, the zinc oxide being possibly added to give porosity to the jelly. The E.M.F. was about 1·3 volt, the internal resistance of different cells of the same size was very different, and the resistance of any one cell varied in an irregular way during working. The cells polarised rapidly when used, and were also liable to short-circuit internally. Nevertheless, their compactness, portability, freedom from all creeping of the salts, and the fact that they did not dry up, led people to consider whether cells constructed somewhat on the principle of the Gassner dry cell might not be manufactured so as to be commercially useful.

Many experimenters attacked this problem; and of the various workers, two of the most successful were Hellesen, in Germany, and Burnley, in America.

141. Hellesen Dry Cell.-In the "Hellesen" dry cell, which was introduced into England by Messrs. Siemens Bros. about 1890, the carbon rod, c, is made hollow and packed with silicate cotton, sc (Fig. 206).

The rod is surrounded with a black paste, м, shown by analysis to be composed mainly of manganese peroxide, broken bits of carbon, water, the oxides of magnesium, silicon, and iron, together with a small quantity of calcium oxide. Outside this is a white paste, L, composed mainly of water, the oxides of calcium and zinc, ammonia, and a trace of magnesium oxide mixed with some gelatinous substance.

Next comes the zinc, z, in the form of a pot, the whole being contained in a millboard case, MC, packed inside with sawdust, s. The top is closed by pouring in melted pitch, P; and a water tube, WT, is inserted to carry off the gas that may be generated.

The E.M.F. of a new Hellesen cell is about 1'45 volt, and for a cell weighing 1 lb. 7 oz. the resistance is about 1 ohm when 1 ampere is flowing. If a cell of this size be used to send a current

WT

MC

of 0.1 ampere, the P.D. falls about 9 per cent. in the first hour, and in each subsequent hour about 3 per cent. more of its original value.

The oxide of iron is used in the Hellesen dry cell probably to assist the manganese peroxide as a depolariser, but several of the other oxides employed in this cell appear to have no use beyond giving porosity to the paste and perhaps absorbing the ammonia gas. The majority of these oxides have, therefore, been discarded in the Burnley dry cell as well as in the form of dry cell subsequently devised by Dr. Obach.

142. Burnley, or E.C.C., Dry Cell. A simpler type of dry cell, devised by Burnley, is sold by the

General Electric Company, London, but is generally called the "E.C.C." dry cell, because it was first brought out by the Electric Construction Corporation. It consists of a carbon rod, c (Fig. 207), surrounded with a black paste, M, composed of manganese peroxide, and powdered carbon, moistened with a solution of sal ammoniac and zinc chloride. This is surrounded with a white paste, L, composed of plaster-of-Paris and flour moistened with a solution of sal ammoniac and zinc chloride. The zinc case, z, is in immediate contact with this paste, and is sealed at the top with a bituM minous compound, P, proper arrangements being made for the escape of gas by the small L tube, w T. The whole is then inserted in a mill-board case, I, which, in the later form of this type of cell, covers the whole of the zinc case, in order to produce good insulation, instead of merely serving as an insulating base, as in the figure.

The E.M.F. of an E.C.C. cell is about 1.45 volt, and a cell will send a current of 0.1 ampere for 200 hours before the E.M.F. falls to 0.5 volt.

weighing 2 lbs. 1 oz.

Tests made by Professor Jamieson* show that an E.C.C. cell polarises less rapidly when sending a given current than does a Hellesen cell of about the same weight, and therefore the E.C.C. cell gives out a larger amount of energy, for a given percentage fall in the terminal P.D.

143. Obach Dry Cell. The Obach dry cell, which was introduced by Messrs. Siemens Bros. in 1894, is * "Proceedings Phil. Soc. of Glasgow," 1892-93.