Bunsen's zinc-carbon battery is a modification of Grove's, the only difference from the latter being the substitution of carbon for platinum foil. The carbon rod or plate becomes brittle in time through the action of the battery, and should therefore not be too thin; this necessitates a much larger porous cell than in Grove's element, and makes the battery more bulky. It is, however, to be preferred to Grove's for (1) it is much less expensive, and (2) owners of Grove's battery experience that the valuable platinum plates offer a bad temptation to workmen, and at times disappear in a mysterious way.

In Fig. 32, A represents a single element of Bunsen's battery, a is the outer cell made of glass, earthenware, or vulcanite, the zinc-plate bent round, with a binding screw, b, at the top. c, a round porous cell, with a wooden lid at the top, through which a carbon stick or rod passes; another binding-screw d is attached to the top of the carbon-rod. The wooden lid at the top is not absolutely necessary; instead of it, a clamp binding-screw may be fixed at the top of the carbon (see B). Carbon is a very porous substance; if the top is not protected, the acid will rise in it by capillary attraction, and soon destroy metallic fixtures by oxidation. For this reason, the top of each carbon plate or rod should, before being first used, be soaked in hot melted paraffin wax.

If artificial carbon is used, it is put in the outer cell and shaped like Z, in Fig. 32. The carbon rod described need not be carefully squared up - it may be of very irregular shape; pieces of the hard coke obtained as scurf in gas retorts are sometimes employed, without being finished up. If a battery is fitted up with such pieces of carbon, the latter should be as nearly as possible of equal size.

Strongest nitrio acid is put into the carbon cell, and acidulated water, 1 to 10, or 1 to 12, into the zinc cell. The action is similar to that in Grove'. cell.

For making, at a low cost, a battery Tor producing electric light, buy 30 empty salt-jars at 2d. each, or 5s. the set; 30 round porous cells, 5 in. high and 2 1/2 in. diam., at 3s. to 4.. per dozen; 30 pieces of zinc, 10 in. by 5 in. by 1/6 in. or 3/16 in., ready cut, at id. to 5d. per lb. Bend the latter round as shown in Fig. 32. by heating them, if necessary, as described on p. 76, and amalgamate them. Cut 29 pieces of copper-wire (No. 18), each 11 in. long. Buy also 30 pieces of carbon rod, 5 in. by 1 1/2 in. by 1 in., ready cut, and with a small hole drilled right through, 1/2 in. from the, top. If necessary, drill the holes in a lathe, or with an archimedian drill. Two plates, each 1/2 in. thick, side by side, act better than a rod of double the thickness. The end piece of copper-wire is then passed through the top of the carbon, and twisted as shown in D. The other end of the same wire is soldered to the top of the zinc plate a. The wire may be coiled round a pencil, as shown at /. The wire connections and the soldered joints should be warmed, and then painted with three coats of Brunswick black, applied hot. The permanent wire connection from element to element described here considerably reduces the work and time expended in charging the battery.

If parts of the battery are to be used at intervals for other purposes, clamps may be preferred; in that case, a strip of zinc is riveted to the top of the zincplate, and clamped to the carbon of the next cell. The clamps required are sold at 4s. per dozen, but they can easily be made by amateurs.

Fig 32.

To make battery clamp-screws, buy 1 lb. flat brass rod, 3/4 in. wide and 1/6 in. thick; 1/2 lb. brass wire, No. 8 gauge; and 1/4 lb. stamped hexagonal nuts. Cut a thread on the wire, and cut it in pieces of J in.; tap the nuts, and either rivet or solder the threaded wire into the nuts; drill a hole at 1/4 in. from one end of the brass rod, and similar holes at 2} in., 5 1/4 in., 7 3/4 in., etc, to the end of the rod, all holes being 2 1/2 in. apart. Tap all the holes. Bend the brass to shape in the vice, as shown in E; cut each clamp off, and lit the screws in. You can thus make your own clamps at about Id. a piece, or less. They are just as good as these made from castings, but do not look quite so well finished. The 30 elements of the battery are connected as shown in F; clamp-binding screws are used for terminals. Cost of the Materials for, a 30-Element Bunsen's Battery for producing Electrics light.

Of course 50 elements will give a better light than 30, and 100 a better one than 50, but a 30-element battery gives a very good light. It should always be borne in mind that one cell gives as much quantity as 50 of the same size, and that doubling the size of all the plates employed doubles the quantity; also that 50 elements will give 50 times the electro-motive force of one element. To produce light requires both as much quantity and as much electro-motive force as it is possible to obtain with batteries; the more of the latter, the longer will the spark be, and the more of the former, the broader the arc. (Wiesendanger.)

" With one exception, Bunsen's is the only real producer of voltaic currents that can be cheaply applied and depended upon in the production of electric light. Its current, once started, is almost constant for about 4 hours, and a good light may, with confidence, be depended upon for 3 hours." (Urquhart.)

An improvement upon the common practice of simply clamping the carbon by a binding-clamp of brass for the connection, is to give the block a heading of lead. Dry the head, cut a notch or two around it J in. from the end; melt the lead, and pour it into some square mould; before it sets, dip in the carbon end; allow to solidify before removal. While still hot, the binding-screw may be soldered on, and, before it cools, the whole should receive a coating of melted pitch; or, better, dip the head in melted paraffin, which, when cool, will defend the connection from attacks of acid. A still better way, although not so quickly accomplished, is to electrotype a heading of copper upon the rods. Partly fill a porous pot with acidulated water; place in an outer cell containing crystals of copper sulphate dissolved in warm water. Heat the rods, and give them a coating of paraffin, driven in with a hot iron, between where the liquid will reach up to and the heading will reach to. If any paraffin spreads upon the end, drive it back by heating; cut a few notches in the head as before, and drill a hole through; in it tightly place a piece of stout copper wire, having 3/4 in. of the end projecting at each side.

Tie a wire around the carbon block; at the end fasten a strip of zinc, and place it in the porous cell, while the carbon head dips into the copper solution. Copper will begin to deposit upon the wire and carbon; when it has attained a thickness of good brown paper, remove the block, drill 2 holes through the copper and carbon, soak a little time in warm water, dry off, and place in melted paraffin. The binding-screw may be soldered to the copper, which will be found of great utility as a heading that is not attacked by acid.

When a " charge " is worked out, the outer acid is exhausted. The nitric acid will have assumed a reddish colour, and may be used again. Next time it turns green, and the third time quite clear, when it should be replaced by fresh.

While at work, this battery gives off fumes of nitric acid, which renders it necessary that it should be placed in a draught. The fumes are poisonous; and are worse while the porous cells are being emptied into the stock bottle.

In pulling the battery to pieces after operations, all connections are first loosened; the zincs are placed in a bucket of water to wash off the acid; the carbons are similarly treated; and the porous cells are emptied into the nitric acid stock bottle, and plunged into water. The outer liquid is thrown away as useless. Porous cells, once used, are kept in water for a few hours to soak out nitric acid or zinc sulphate. All connections are washed and dried, and examined for oxidized or bad-contact points, which must be scraped bright or filed.

The fore of the Bunsen increases after setting up for about an hour, and the full effect is not attained until the acid soaks through the porous cell. Carbons are not affected, and last any length of time. The zinc is slowly consumed, through the mercury coating. 25 Bun-sen cells will give a very brilliant light, and 50 will produce an arc of great power. The conducting wires must be about No, 12. (Urquhart and Webb.)