Bichromate batteries of bottle shape as in Fig. 29, with 2 carbon plates, a sliding rod and movable zinc plate, are very extensively used by experimenters and lecturers, because they are always ready for being put to work with one motion of the hand, not necessitating any other preparation; and as soon as the desired result is obtained, the battery can be put out of action with the same facility. a is the bottle; b, a brass cap for the top; c, a disc of ebonite, to which the carbon plates are fastened; d, a binding-screw, connected with the 2 fixed and parallel carbon plates, be-tween which is suspended a zinc plate of about half the length. This plate of zinc is fixed on a brass rod, whose extremity is shown at /, made to slide up and down in a brass tube, which is either close-fitting and split, or loosely fitting and provided with a set-screw. In either case, it must be connected, by means of a copper band, with the terminal e. The excitant is a saturated solution of potash bichromate, to which is added 1/10 volume of sulphuric acid; another 1/10 should be added after the battery has worked for some time, when it begins to become weaker in action.
For all purposes whore a strong current is required at intervals, such as the working of induction coils, medical coils, small electromagnetic machines, and laboratory experiments, this battery is by many preferred to all others. The prices of such batteries range from 6s. 6d. to 24s., according to the sizes and number of the plates, 3 carbons and 2 zincs being employed in the largest apparatus of the kind. Amateurs can hardly make this kind of battery so cheaply as it can be bought, because the bottles are high in price unless ordered in large numbers at a time.
For this reason, Wiesendanger has devised a form of bichromate battery having all the advantages of the one described above, and which can be made by amateurs at a fraction of the prices quoted. To make such a battery, take a marmalade jar g, measuring inside about 5 in. by 2 1/2 in. diameter at the top. Get 2 plates of carbon, each 5 in. by 2} in., and a zinc plate (with terminals) of the same dimensions. Cut 3 pieces of thin mahogany board, 3 in. by 1/2 in. by 1/6 in., and a clamp c, such as shown in Fig. 30. The clamp c should have a ring a, at the top, bo that it can be suspended from the hook b, when the battery is not in actual use. The 2 pieces of wood d are put right and left to the top of the zinc plate f, as shown, the 2 carbons e to the right and left of the pieces of wood, and the whole arrangement is clamped by means of the top-clamp c. The frame can be made with a piece of bent brass strip h, 9 in. x 1/2 in. x 1/16 in., to which the upright brass rod i is soldered; or it can be made of stout brass wire, No. 7, altogether. In either case, there will be so much elasticity in the ring A, that it can easily be put on, and will hold firmly on the top of the jar.
This frame can be bought ready made for lOd When the battery is to be used, the plates are inside the jar and in the liquid, supported by the projecting ends of the wooden pieces d; when out of use, the plates are lifted and suspended from the hook b, by means of the ring a, at the top of the terminal clamp. (Wie-sendanger.)
Trouve has considerably improved the bichromate battery by supersaturating the exciting fluid. He takes 21 dr. of potash bichromate powder to 1 pint water, and adds, after shaking, drop by drop, 63 dr. sulphuric acid. The liquid warms, and-the salt dissolves. No crystals form on cooling, nor are chrome-alum crystals deposited in the cell. With 12 elements and the foregoing solution, 10 incandescent lamps can be kept going for 5 hours, each lamp being equal to 10 candles. The electromotive force of the cell is 2 volts with fresh solution, and the intensity of the current at the beginning on a short circuit is 118 amperes. The resistance is 0•07 ohm; 4 batteries working a Gramme machine have produced 14 kilogrammeters of work during 3 hours without weakening in power.
A test of Tronve's battery has been made by Hospitalier with 2 sets of 6 cells each. The cells are of ebonite, and are arranged in a box or trough in such a manner that they can be easily removed-one aide of the trough being hinged so that the cells can be slid out when the plates are drawn out of the solution. The. elements are connected in series, the zinc of one cell being coupled to the 2 carbons of the next, and a winch and ratchet enables the plates to be wound up out of the solution, or to be immersed to any desired extent. The ebonite cells are no larger than is absolutely necessary, and the quantity of solution required for a battery of 6 is given by Hospitalier as 17.6 lb. water, 2.64 lb. powdered bichromate of potash, and 8 lb. sulphuric acid. This gives 5 1/4 oz. bichromate per 1] pint of water, or a solution half as strong again as that used by Poggen-dorf. Trouve uses a much stronger solution on certain occasions. Care must be taken not to use a wooden stirrer, as that would be carbonised, to the injury of the solution. The total weight of a battery of 6 cells is said to be 74 lb.
In the eiperiments made by Hospitalier, 2 batteries of 12 elements together were connected in series, and used to work 6 Swan lamps, the difference of potential at the terminals being, batteries 16.70 volts, lamps 14.15 volts. That was during the constant phase, for at the moment of immersion of the plates, there is great initial E.M.F. which produces a current of 12 amperes, and the object was to maintain a constant current of 8 amperes. The zincs were mmersed at first to about 4/3 in., and after a few minutes the current reached its normal strength of .8 amperes. After 1/4 hour the batteries had settled to steady work, and for 1} hour the zincs were not lowered. After that time, the decrease in the delivery was compensated by gradually increasing the immersed surface. By this means, for nearly 4 1/2 hours the variation did not exceed 1/2 ampere, but after that time the zincs were immersed to the full extent of nearly 6 in, and the current gradually fell, until, at the end of 5f hours, it had reached 5 amperes, and the experiment ceased. The result shows that each battery practically furnished 1/2 h.p.-hour, the consumption of zinc being 365 dr., bichromate 600 dr., and sulphuric acid 1800 dr.