It will likewise find an application in laboratories, where the bichromate pile is in much demand because of its powerful qualities, and where it is often necessary to control it from quite a distant point. ( La Nature.)
The battery consists of a series of carbon and zinc plates, presenting a very large surface, which is exposed to the action of a solution of bichromate of potash with sulphuric acid. Each plate is rectangular in shape, as in Fig. 73, but each has one of its top corners cut away. The zincs, as in Fig. 73, are deficient at the upper right-hand corner, and the carbons, as in Fig. 74, are cut away, corresponding to the left upper corner. In this way space is left to join each series of plates by a strong brass rod provided with washers, by which their distance apart is regulated. The rods are joined to a common handle above by metal pieces, care being, of course, taken to secure the insulation of one series from the other.
Jack for immersing zincs.
Fig. 73 represents the buttery arranged in its most compact and portable form, the reservoir being an oak box lined with sheet lead. The zinc and carbon element) are lifted in and out of it by means of the wooden handle A, by which they are all tupported, and which rests by its ends in grooves cat at each end of the box. When required for use, the bichromate solution previously prepared is poured in from a bottle. After use, the plates are raised out of the solution by means of the handle, washed by dipping into a large vessel of water, and replaced in the box, the contents of which have been previously emptied Fig. 74 represents the battery as arranged for ordinary use. The reservoir contains a large quantity of solution in which the carbon plates are immersed to 1/5 of their height. The zinc plates are lifted entirely out of the solution by the action of the iron weight B, which more than counterbalances them. The drawing represents them in process of being lifted out. It fails to show that the connection of this bar with the zinc series is made by arms of metal outside the box. When required for use, the bar weight B is lifted by hand,the zinc plates dip into the solution, and ultimately pass into the narrow spaces which are interposed between each pair of the fixed carbons.
If the battery be required to act for any length of time, the solution is effectually agitated and polarisation prevented by an occasional elevation and depression of the weight.
The internal resistance of this battery is very small; its power of overcoming proportionately little, and accordingly only 30 silk-covered ropes of copper wire are necessary in connecting it with the platinum to be heated. (C. Coppinger.)
(a) Prescott describes a chromic acid battery where bichromate of potash and sulphuric acid take the place of nitric acid in the Bansen battery. The solution is made by dissolving 1/2lb. bichromate of potash in 51b. hot water, and when cold, adding strong sulphuric acid. The zinc is placed in the outer jar containing a saturated solution of common salt, which is made by adding salt until the water can dissolve no more. Five of these cells give a fair current to a small lamp, but the porous pots are soon ruined. - sometimes the bottom coming completely off. But to make a cheap battery of this description fur bells, the elements mast be reversed. Id the outer jar should be a Urge surface of carbon in bichromate solution (adding a few drops of sulphuric acid), the lumps of graphite packed closely round the porous pot. Inside the porous pot a strip of zinc is placed in a solution of salt. Two of these cells coupled for intensity will be sufficient to ring any ordinary bell. If yon do not wish to spoil the porous pots, place just enough salt to acidulate the water - say 1/2 teaspoonful to each. The bichromate will last a long while; but the saline solution in the porous pot should be changed once in 6-8 weeks.
To keep a couple of porous pots in clean all the impurities would soak out in the mean time. A couple of these cells can easily be made up by experimental electricians for next to nothing in cost, as many hare most of the materials required at hand.
Battery for cautery.
Battery fur cautery.
(b)'Fig. 75 is better than the ordinary compact, and giving a greater current. This battery is very constant, and will light a lamp for many hours. The cell may be used with the usual chromic-acid solution. E.M.F. 2-03 volts. Internal resistance for 4 sq. in. zinc surface = 2/3 ohm; copper-wire to be attached to the binding-screw A of next cell. This wire may also be used to suspend the elements. To keep the carbon rods at a suitable small distance from the zinc rod and from each other, small pieces of rubber tubing are slipped on each rod at the two ends, and are seen in the sketch partly covered by the elastic bands, above which the carbons are coated with copper by electrolysis, and have soldered to them a flexible ligament of fine copper wire, and other wires (A and B) for supports to suspend the elements in the neck of a bottle. The ligament is cat through between the rods marked 2 and 3, so that when the two elastic bands have been removed the carbons may bo detached from the zinc. The above is the arrangement of carbon and zinc elements for one cell, the carbon being in the form of rods.
Fig. 76 is a modification of the standard gravity battery. The sulphate of copper is enclosed in the straw-board box, the zinc in a paper envelope, and the rim of the jar has attached to it on the inside by a sticky substance a band of rubber cloth. The advantage! claimed for the cell are at follows: - When the battery is first set up, the dust of sulphate of copper is not instantly dissolved and diffused throughout the liquid, coating the zinc with copper, as is the case with the ordinary cell. Several minutes elapse before discolouration of the fluid begins, and then only at the bottom of the cell, from whence it rises very gradually, never reaching the sine If the box be filled, the charge of copper is always uniform. Deposited or spongy metallic copper cannot fall upon the crystals from the zinc, and caking or massing in the bottom of the jar is thus prevented. The copper electrode is held firmly in position, always the same distance from the zinc In a battery of any considerable number of ordinary celts it would be difficult to find two alike in this respect. There are little or no stalactitic formations from the zinc, and conse-quently no local action, rendering the battery very useful for open-circuit work.