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The Gravity Battery

In consequence of the trouble caused by the precipitation of the copper on the porous cell in the Daniell battery, Cromwell F. Varley, in 1854, while experimenting, found that the difference in specific gravity between solutions of sulphate of copper and sulphate of zinc was sufficient in itself to entirely separate them, the copper solution lying at the bottom of the cell, and the zinc solution remaining superposed upon it. He accordingly dispensed with the porous cup, placed his copper element at the bottom, and the zinc near the top of a glass jar, and thus originated the gravity battery of to-day. It is the simplest, most reliable and constant form known, and has displaced all others for closed circuit work, requiring a low voltage, such as telegraphing, etc. Its voltage, when first set up, is 1.07, running down under constant work to .90, and a gallon cell will give one half ampere on short circuit. The form of cell shown in Fig. 119 is known as the "crowfoot," on account of the manner in which the zinc (positive) element is spread out, to expose a large surface of zinc to the solution. It is the form used for telegraphing, and, therefore, can be readily attained anywhere. Of course, other forms, shapes and sizes can be made at the option of the workman. To set up this battery, the copper strip, being unfolded so as to form a cross, is placed at the bottom of a jar, the zinc is suspended from the top as shown, and clean water, containing one-tenth of a saturated solution of sulphate of zinc is added, until it nearly touches the zinc. Sulphate of copper crystals are then added one by one until, if the battery is meant to be continually used, they nearly cover the top of the copper strip. If the battery is not intended for continual use, it will be found more advantageous to use but a few ounces of sulphate of copper, as the more concentrated the solution, the greater is the tendency to local action. The sulphate of zinc may be dispensed with if the battery is not required for immediate use; in this case, the latter should be short circuited, and left so for several days. The need of blue vitriol will be indicated by the discoloration of the lower stratum of the liquid. It is best to keep the line marking the two solutions about halfway between the zinc and copper. Should the sulphate of zinc become too concentrated, a portion of it should be removed by means of a syringe or cup, and its place supplied by water. To determine when this is necessary, a hydrometer may be used. Below 15 degree the solution is too weak; about 25 degree it is too strong, and should be diluted. If the battery is taken care of from month to month, it should not require a thorough cleaning more than once a year. When this is done the deposits formed upon the surface of the zinc should be scraped off, the jars washed and the liquids renewed as in the beginning.

If, however, the batteries are in constant use, care must be taken to keep the zincs clean and the solutions as indicated above. If the sulphate of zinc is allowed to become saturated, it will crystalize on the zinc and on the edge of the jar, gradually creeping over the edge. This should be wiped off with a damp cloth and a little oil or fat smeared over the top of the jar to prevent creeping. The jars should not be disturbed, as this would cause the two solutions to mix, and they should be kept in a dry, even temperature, (60 deg. to 80 deg. F). Freezing would stop the action of the battery. The Bunsen, or Carbon Battery. Fig. 120, consists of a glass jar containing a hollow zinc cylinder, slit on one side to allow a free circulation of the solution; within this stands a porous cup containing a bar of carbon. To charge this battery, the amalgamated zinc is placed in the glass jar, the porous jar in the center of the zinc cylinder and the carbon in the porous jar. In the outer jar is sulphuric acid, diluted with twelve times its weight of water and in the porous jar electropoion fluid. (See Electropoion Fluid, page 143).

The voltage of this battery is 2.028; its amperage cannot be given, as it depends largely upon the care which is given the battery, the size of the cell and the condition of the porous cups, which vary greatly in porousity and conducting power. It emits fumes of hydrogen and sulphurous acid if not in good condition, and should not be used in the same room with fine tools or metal work that is liable to injury. It soon runs down, requiring recharging every day when in constant use, but is simple to handle when understood and is generally furnished in small outfits for nickel plating, etc., on account of its high voltage and the quantity of current given off when in good order. The zincs must be kept well amalgamated or they will polarize very rapidly and destroy the current; care should also be taken that no sediment be allowed to accumulate in the porous cup and fill its pores, thus stopping the action. It is more expensive to run than the gravity, as the zincs are eaten by the acid much faster, especially if not kept well amalgamated; but it will deliver a greater quantity of current in a given time than a gravity cell of equal size. The internal resistance of a new cell is about one-half an ohm. The plates should be removed and cleaned when the battery is not in use.