production of the zinc sulphate, and of the water, on the passage of the current, both lowers the E.M.F. and increases the resistance of the cell, so that the current which a Grove's cell can send through a small fixed resistance falls off considerably, even after an hour or two. When the specific gravity of the nitric acid is reduced by the production of the water to 1.33, and the dilute sulphuric acid has a specific gravity of 114, the E.M.F. of the cell falls to about 1.8 volt. Further, as the nitric acid becomes more and more dilute the nitric oxide is no longer converted into nitric peroxide in accordance with the last equation given above, and the reddishbrown colour of the liquid turns to a greenish-blue. A Grove's battery must be taken to pieces at the end of each day's use, since the mixing of the liquids through the walls of the very porous pots, used to separate them, would render the battery practically useless the next day. The porous pots should be placed in water and left to soak all night, so that all the zinc sulphate solution may be dissolved out of the pores of the earthenware, for, otherwise, when the pots are dried the zinc sulphate solution will crystallise in the pores and cause the pots to tumble to pieces. If we take the E.M.F. of a Grove's cell as 1.9 volt, and its resistance as 0·15 ohm, then a battery of Grove's cells in series will produce a current of a little over 6 amperes when the external resistance is equal to that of the battery-the condition (as was explained in § 124, page 392) that leads to the external circuit receiving the maximum power from a given battery. And in each hour in each cell with this current of 6 amperes about 0.26 oz. of zinc, 0.77 oz. of nitric acid (sp. gr. 1.4) will be used up; 0.39 oz. of sulphuric acid, and while 0.64 oz. of zinc sulphate in solution, 0.14 oz. of water will be formed, without any allowance being made for waste of material through local action. Example 118.-—If 4 lbs. of zinc have been consumed in a Grove's battery, how much sulphuric acid has been used up, assuming that no local action has taken place? Answer.-6 lbs. Example 119.-25 Grove's cells in series are sending a current of 81 amperes. In what time will 2 lbs. of nitric acid be consumed? Answer. Since about 0.128 oz. of nitric acid (sp. gr. 1.51) is consumed in a Grove's cell per hour per ampere, the time required will be about. hour 11 minutes. 2 × 16 or about 1 Example 120.--A battery of 12 Grove's cells in series, when sending an average current of 4 amperes for 6 hours 40 minutes, consumes llb. of zinc. What percentage of the zinc is wasted in local action ? Answer.-16.2 per cent. Example 121.-A Grove's cell, having an E. M.F. of 1.9 volt and an internal resistance of 0·1 ohm, is used in series with a Daniell's cell, having an E.M.F. of 1·1 volt and an internal resistance of 3 ohms, to send a current through an external resistance of 2 ohms. Calculate the power in watts developed by each cell and the power wasted in heating each cell. Answer.- Power developed by the Grove's cell equals 1.117 watt. Example 122.-If zinc costs £18 a ton, sulphuric acid 1d. a lb., and nitric acid 3d. per lb., what is the least cost of supplying a Board of Trade unit of energy to a wire of 3 ohms' resistance by means of 20 Grove's cells in series, each having an E.M.F. of 1.9 volt and a resistance of 0.15 ohm? Answer. The current equals 20 × 1.9 or 6.333, amperes, therefore the power given to the wire equals (6.333) × 3, or 120-3, watts. Consequently, the current. 1000 must flow for or 8.313, hours in order that the 120.3' wire may receive a Board of Trade unit of energy. Hence, the zinc consumed will cost about 5 d., the sulphuric acid about 6d., and the nitric acid about 2s. 5d., or about 3s. 5d. altogether, when no allowance is made for waste of material through chemical action. Example 123. In the preceding question, how many Board of Trade units of energy were actually developed by the battery, and what was the cost of developing the Board of Trade unit? Answer.-2; for, since the resistance of the battery was equal to that of the external circuit, 1 Board of Trade unit of energy was wasted in heating the battery, when 1 Board of Trade unit was given to the external circuit. Hence, the minimum price of the chemicals consumed in a Grove's battery when 1 Board of Trade unit is developed altogether is about 1s. 9d., when zinc, sulphuric acid, and nitric acid can be bought at the rates mentioned in example 122. Example 124.-If the resistance external to a Grove's battery be very large compared with that of the battery itself, what will be the least cost of supplying a Board of Trade unit of energy to the outside circuit, with the prices of materials mentioned in example 122? Answer.—In this case practically the whole of the energy equivalent to that of the chemicals consumed in the battery will be given to the outside circuit; hence the cost of 1 Board of Trade unit of energy given to the outside circuit will be about 1s. 9d. 137. Bunsen's Cell. The "Bunsen's" cell differs from the Grove's only in the platinum plate being replaced by a cylinder, or by a block of carbon shown by c in Fig. 198, which represents a common form of Bunsen's cell much used on the Continent. Its E.M.F. is practically the same as that of the Grove's cell. Carbon is a much cheaper material than platinum, but, as the nitric acid soaks into the carbon, more of it must be used to fill a cell if the negative plate be carbon than if it be composed of platinum. Hence, the first cost of a Bunsen's cell is less than that of a Grove's, but the cost of working it is greater. If, then, the cells are to be frequently employed to send a current it is more economical to purchase Grove's cells, whereas if they are to be only used occasionally it may be cheaper, on the whole, to obtain those of the Bunsen type. The carbons for the Bunsen's cells are either cut out of retort carbon, or are made by baking in a furnace fine coke-dust and caking coal in an iron mould; then, in accordance with a process invented by Bunsen, the baked mass is soaked repeatedly in thick syrup or gas-tar, and re-baked to impart solidity and conducting power to it. 138. Potassium Bichromate Cell. This is a form of cell devised by Prof. Poggendorff, in which the depolariser is chromium trioxide (Cro), popularly called chromic acid, since chromium trioxide dissolved in water has a strong acid reaction. But, as the chromium trioxide used formerly to be prepared, by the user of the cell, by acting on potassium bichromate, K,Cr2O7, with sulphuric acid the cell is frequently called the "potassium bichromate" cell. Now, however, crystals of chromium trioxide containing 5 per cent. of water of crystallisation can be purchased ready prepared, and when these are used the cell may be shortly called a "chromic acid" cell. The cell is constructed in two forms, one without and one with a porous pot, seen in Figs. 199 and 200 respectively. The plates employed are of carbon, K, and amalgamated zinc, z (Fig. 199), two carbon plates being generally used with the former type of cell to diminish its chromate Cell without Porous Pot. resistance. The zinc plate z is sup- Fig. 199.-Potassium Biported by the rod a, and should be pushed into the liquid only when the cell is required to give a current, and withdrawn directly the current is interrupted, otherwise an insoluble chromium salt forms on the surface of the zinc and interferes with the action of the cell. The chemical change which takes place when a current passes through a single fluid chromic acid cell, containing chromium trioxide dissolved in dilute sulphuric acid, is as follows: Before sending a current k(C)+(CrO3)+m(H2SO4)+n(Zn), k(C)+(l−2)(CrO3)+(Cr23SO4)+(m-6) (H2SO4)+6(H2O) +3(ZnSO)+(-3)(Zn), |