A galvanic battery consists of 2 electrical conducting bodies placed in a fluid which will exert greater chemical action on the one (the " positive element") than on the other (the "negative element"). The substances commonly used as elements are zinc opposed to carbon, cast iron, copper, platinum, and silver; while the usual chemical exciting fluids are nitric acid, sulphuric acid, copper sulphate, and potash bichromate. The positive element of the battery is at the same time its negative pole (or cathode), the former term applying only to the portion of the plate within the exciting fluid, while the latter denotes the portion outside the fluid. In the same way, the negative element is the positive pole (or anode).
The continuous arrangement of 2 or more cells, as just described, in a set, constitutes a compound battery. There are two ways of arranging the set, according as quantity or intensity of current is desired. In the former case, the anodes of all the single cells are attached to one wire, and the cathodes of all to another: the effect is then equal to the product of a single cell whose element surface corresponds to the sum of the surfaces in the single cells. To increase the intensity of the current, the anode of one cell is united to the cathode of the next, throughout the series, so that the current from each cell circulates through all. Other conditions being equal, the power of a battery is dependent upon the area of its negative element; and in making an intensity arrangement, it is very important that the negative elements of all the cells shall be of equal area, as the power will be regulated by the smallest negative element in the series.
'Che anode of a quantity battery is any part of the wire to which the anodes of the single cells are joined, the cathode in like manner being any part of the wire receiving the individual cathodes; but the anode of an intensity battery is the unconnected anode of the last cell of the series, and its cathode is that of the first cell. To apply a battery, the current is conveyed from each of its poles by means of electrically conducting wires arranged without interruption.
These generally constitute the positive elements by whose oxidation the current is produced. The best kind is "roiled Belgian," about 3/16 in. thick, and costing something like 4d. a lb. New plates are coated with a greasy film, which needs dissolving off with caustic soda solution. This done, pieces of the required size are cut off by making a deep incision on each side, and letting mercury rest on the line, which it soon penetrates sufficiently to facilitate breaking the plate by bending over an edge. Cylinders may be formed by placing the plate in hot water, and curling it round a wooden roller.
The plates for some batteries (e.g. Bunsen's, Callan's, Grove's, Smee's), used to be amalgamated with mercury when the battery is made, and re-amalgamated subsequently at intervals. The amalgamation is performed by laying the plate in sulphuric acid of the same strength as the exciting fluid, and allowing it to remain some minutes. It is then taken out, and, while wet, has a globule or two of mercury rubbed over its face and edges, so that it may present a uniform silvered appearance. This is to prevent " local action:" its necessity may be thus exemplified. If a piece of unamalgamated zinc be put into dilute sulphuric acid, the surface will be immediately covered with minute bubbles of hydrogen, and the acid solution will soon become in a state of effervescence. This will go on until the zinc is dissolved, or the solution is saturated with zinc sulphate. On the amalgamated plate no action of this kind will take place; the smooth ness of surface communicated by the mercury appears to cause the hydrogen first evolved to adhere to the plate, which is thus protected from any further action. This immunity only exists while the plate is detached from the other part of the battery, or the circuit between anode and cathode is severed.
When the plate is in place, and the circuit is completed, the mercury affords no protection, as the hydrogen then is evolved at the negative element. When zinc plates, after being washed, look black in parts, they need re-amalgamating. (Dyer.)
The prepared plate is provided with one of the various forms of binding-screw, or has a strap of sheet copper, 5 in. long and uniform in width, soldered to it. The attachment is best effected by drilling a hole through both plate and strap, and firmly riveting; the strap must be clean at the joint, and the latter may be preserved from the action of the acid by a coat of varnish. For soldering a joint, use soft solder, zinc chloride solution for the flux (or rosin for copper to copper or brass), and a " tinning iron."
These vary with the kind of battery, as described hereafter.
The same remark applies here.
Obviously the positive and negative elements of a battery must not be in contact within the exciting fluid; they should be separated by a space of 3/8 to 1/2 in. In the case of batteries without porous cells, periodical attention will need to be given to ensure this condition being maintained.
Containing cells are made to hold 1/2 pint to 1 gal. Single-liquid cells have but 1 containing vessel; double ones have 2. Outer cells are made, as a rule, of glazed earthenware, though glass may be used; porous cells, of un-glazed earthenware. The latter are put inside the zinc cylinder, or U -plate, and generally contain the positive element. Such cells should not be too hard and dense, nor the thickness of the sides exceed 3/16 in. The softest are of red ware; but better cells, and sufficiently porous, are made from white clay. A test of the porosity is made by placing water in the cells, and allowing them to stand for about 15 minutes: if then no dew appears on the outside of the cell, it is probably too hard or thick, and will offer too great a resistance to the current; while if the water actually runs off the side, the cell is too porous, and will shorten the period of action of the battery by too rapid transfusion of the liquids into each other. One-liquid cells, though convenient for short experiments, rapidly acquire a film of gas upon their negative plates, whereby the development of the current is impeded; consequently such cells, unless the excitant is agitated in some way, are unfit for supplying current for a length of time.
This fault is termed "polarization." In two-liquid cells, the negative plate. is surrounded by a liquid rich in oxygen (e.g. nitric acid in Grove's), which absorbs the hydrogen liberated at the negative plate, and keeps the latter free from film. But two-liquid cells are more troublesome. (Urquhart and Webb.)
Following are the most common forms of battery.
Antimony is precipitated on the carbon cells described on p. 84, and in conjunction with these are used unamalgamated zinc rods and a solution of ammonium chloride. Such a battery may be useful for general laboratory purposes, being somewhat similar to Smee's in action, but with much cheaper negative elements, and a more economical utilization of the zinc. The carbon cells can be made small enough to be nearly close to the zinc, while, by having the containing vessels of ample size, the constancy of the current is enhanced. .
Another form of this battery may be made with larger carbons and porous diaphragms, using for the outer liquid a mixture of antimonious chloride and ammonium chloride.
Plaster of Paris diaphragms can easily be made with dry plaster in the following manner:- Provide an inner core or mould of turned wood, a little tapering, and with a shoulder the thickness of the required diaphragm; round this tie 2 thicknesses of stout blotting-paper, well shake down the dry plaster between the wooden core and the paper, and immerse the whole in water. In a few minutes the diaphragm will be solid, and can be removed from the mould. (Symons, Rep, Brit, Assoc.)