Donald M. Bliss.

The batteries most in practical use may be divided into two classes - single fluid and double fluid cells. The single fluid type consists of two plates of dissimilar metals, or one metal and carbon separated from each other, mounted on a suitable support, and arranged to dip into a jar containing an alkaline or acid solution. A galvanic battery is the name applied to a number of such single cells joined together so as to add their pressures or capacity together. The electrodes or poles of the battery cell are the terminals or binding posts connecting the ends of the plates. In connecting up a battery, it should be remembered that the polarity of any given electrode or binding post is always of the opposite sign to the plate with which it is connected. For instance, in the case of a zinc and carbon cell, the binding post or wire connected to the zinc will be spoken of as the negative pole of the cell, while the zinc itself is the positive element. In these articles, when the term "positive" or "negative "is used in connection with batteries, the polarity of the binding post or wire connecting to the same is always intended. In considering the substances most available for the formation of galvanic cells, the following table includes the most prominent members : 1, zinc ; 2, cadmium; 3, tin ; 4, lead; 5, iron ; 6, nickel; 7, bismuth; 8, antimony; 9, copper; 10, silver; 11, gold ; 12, platinum ; 13, graphite or carbon. Any two of these metals will form a galvanic cell or couple, and produce a difference of electric potential when immersed in a suitable solution, the first one in the list being the positive element to the next one following, which of course is negative.

If, for example, zinc and copper are used, the zinc will be acted on and will form the positive element. The greatest difference of pressure or potential will be developed between Nos. 1 and 13. For experimental purposes, a battery is required which will yield a great amount of current, together with the greatest amount of pressure per cell, and also be able to deliver its output continuously until the solution is exhausted. For the amateur, the type of cell which most nearly fulfills these conditions is known as the bichromate single fluid cell. Fig. 4 shows its construction very clearly, and it consists of a zinc and copper plate screwed to the opposite sides of a wooden cross-bar. Any suitable form of jar may be used, but fruit jars or large glass tumblers of about one pint capacity will be found the cheapest and most convenient. They may be purchased for a few cents each at any corner grocery. The wood cross-bar, it will be noticed, is notched so as to rest firmly on the edges of the bar. The zinc plate should be drilled with two holes, and screwed directly to the cross-bar by small round-headed brass screws, with a copper or brass washer or burr under each screw to give the wire to binding screw a good connection with the plate. The carbon plate must also be fastened to the cross-bar by two similar screws, but care should be taken that the screws of the two plates do not meet and make contact within the cross-bar. This may readily be avoided by not setting the plates exactly opposite each other, but mounting one plate about half an inch to one side of the other, so that the screws will be staggered and cannot touch. Care should be taken in drilling the carbon plates, as the substance is brittle, and if the drill is forced it is liable to crack the plate. On the top of the cross-bar, at each end, should be driven two more similar screws with two washers under the heads, as shown, and a short piece of copper wire should be connected around the head of one screw on the zinc and under the head of the connecting screw on the top bar. A similar wire should be run from the. carbon plate to the remaining binding screw, as shown. This arrangement is preferable to connecting the wires directly to the plate. After the plates have been mounted in this manner, the cross-bar and the tops of the plates should be dipped in hot melted paraffin for about one inch of their length from the upper end, as shown, and they should be left in the paraffin long enough for it to thoroughly soak into the pores of the wood and carbon. This forms a protection against the corrosion of the acid, which will take place around the screws or in the plates if they are not thus protected. The plates when mounted in position on the bar should reach to within a quarter of an inch from the bottom. The solution used in this form of battery should be made as follows : Take 1 lb. of commercial sulphuric acid. Pour it slowly into one gallon of water, stirring constantly with a glass strip or rod. Some heating will be noticed when the acid is first put into the water. Do not reverse the process and pour the water into the acid, for excessive heat and probable burning will result. Sulphuric acid is a powerful irritant, and care should be taken not to get it in contact with the skin or clothing. After the acid has been added to the water, add twelve ounces of powdered bichromate of potash, and when the solution has cooled, it is ready for use.

The solution may be left in the battery jars, but the plates should be removed when the battery is not actually in use. Before the zinc plates are fastened to the cross-bar, they should be thoroughly amalgamated with mercury. This may be readily done by pouring a small amount of mercury in a saucer or an earthenware dish, and nibbing it on to the zinc plate with a cotton swab, moistened with a solution of sulphuric acid and water. It may take a few minutes for the mercury to start the action, but when once under way it will spread rapidly over the surface of the zinc and form a uniform layer. Both sides of the zinc and the edges should be thus treated, and this process should be renewed when the zincs become badly coated or dirty from continued use. Before starting on the construction of the battery and experimental apparatus described, the student should provide himself with the following list of supplies : 2 lbs. of sulphuric acid, commercial strength, costing from 5 to 8 cts. per lb., 1 lb. bichromate of potash, at 15 to 18 cts. per lb., 1 gross round-head brass screws 3/4" long of No. 8 or 10, a few dozen copper burrs or washers, for fitting the screws, 4 lbs. No. 20 single cotton covered magnet wire, 6 zinc plates 5" x 2", 1/16 to 1/8" thick, costing about 12 cts. per lb., 6 carbon plates same size, costing 8 to 10 cts. apiece, a few lbs. of paraffin wax, and 6 one-pint glass jars. For temporary use, ordinary sheet zinc such as is used by plumbers may be taken instead of regular battery zinc, but the latter is by far the more preferable, on account of its greater thickness and purity. Twenty-five or 50 ft. of No. 14-annunciator or bell wire should also be obtained for convenience in making the various connections between battery and pieces of apparatus.