so much does the conducting power of the negative n.etal affect the practical usefulness of a battery, that notwithstanding the fact that platinum is much more negative than copper, there is so much of the effective electricity expended in overcoming the resistance which the inferior conductibility of the platinum offers to the progress of the current, that a battery of zinc and copper proves to be a more effective and useful battery for electro-metallurgy than one made of zinc and platinum. Hence also the reason that iron and copper, or iron and any other metal, make but an indifferent battery -iron being a bad conductor; while lead, which will be seen in the table, stands lowest in this property, is therefore unfit for batteries.

In fitting up a voltaic arrangement with a negative metal that is not a good conductor, such as platinum, the closer it is placed to the exciting liquid, in connection with another metal that is a good conductor, the better; because the current obtained will be the more effectual.

The following experiments will illustrate these remarks with a few of the common metals used as negative electrodes. There were, in each battery, six square inches of each metal exposed to the action of the acid, which was sulphuric acid diluted with 25 parts of water. The poles were of the same size, of copper, placed in sulphate of copper; and the quantity of copper deposited was taken as the data, each trial being of a different length of time:

FIRST, IN ACTION HALF AN HOUR.

SECOND, IN ACTION TWO AND A-HALF HOURS.

From these few experiments it appears that tin and zinc, when used for a long time, constitute a very effective battery. It is very constant in its action, and thus suited for time. It stands next to platinized silver. The whole, in nineteen hours, gave respectively

BABINGTON'S BATTERY.-If we look back to the description given of the voltaic pile (page 489), and the improvement made upon it by Cruikshanks, we perceive the relation they bear to the pieces of copper and zinc mentioned in page 510; but the relation is mor apparent in Babington's improvement upon Cruikshank's battery When working with this battery, it was found that the energy o Figs. 545

546.

was

Fig. 517.

the battery did not depend, as was supposed, upon the extent of surface of the zinc and copper which were in contact, but upon the extent of surface of these metals in contact with the liquid with which the battery was excited; and that it was sufficient if the zinc and copper touched each other in a single point;-provided that the plates were plunged into the liquid, and that the copper plate should be exactly opposite to a zinc plate in the same cell, a space between them. Hence, instead of soldering the zinc and copper together, as Mr. Cruikshanks did, it enough to effect a communication by turning over a portion of the copper plate at the top, and soldering it to the upper extremity of the zinc. Thus-c, the copper, is bent over to touch and be soldered to the zinc plate z. For this arrangement, the wooden trough was divided, by plates of glass or varnished wood, into as many cells as there were pairs of zinc and copper. The cells being filled with the acid, or exciting solution, the metals

were then placed into them in such a manner that each pair of

zinc and copper plates had a partition between. By this arrange ment the zinc of one pair faced the copper of the next pair in the cell, as shown in Figures 546 and 547. The former represents the plates immersed in the solution; the latter, the plates suspended on a rack over the solution. This arrangement was termed Babing. ton's Battery.

WOLLASTON'S BATTERY.-Although we have spoken of the great value of amalgamated zinc for batteries, still at the period when the arrangement just described was introduced, amalgamation was not known; and the zinc plates were, therefore, always liable to be destroyed by the acid. It was, consequently, of importance that no zinc should be exposed to the action of the acid that was not calculated to give electricity, as the energy of each pair of plates depends upon the extent of surface of the two metals exactly opposite to each other. It will be evident that in Babington's arrangement only one side of the zinc was effective in giving elec tricity, while both sides were exposed to the action of the acid. To

Fig. 548.

its quantity without further cost. 548) shows the manner in which structed.

obviate this defect, Dr. Wollaston caused the copper plate to surround the zine, by which the whole surface of the zinc exposed to the acid was made effec tive in producing electricity, and thereby doubling The accompanying figure (Fig. this battery was originally con.

This improvement, if we except several modifications of con struction for the facility of taking the plates asunder for cleaning, etc., did all for this kind of battery that could be done.

MODIFICATION OF WOLLASTON'S BATTERY NOW IN USE.-Wol laston's battery is still generally used in large factories for deposit ing metals; and it is found by experience to be the most convenient and economical of all the batteries yet contrived. The modifica tion we have found to be very suitable, and practically useful, may be thus described. In the arrangement represented above, when amalgamated zincs are used, small quantities of amalgam fall from the zinc plates upon the copper, which not only occasion local action, but the mercury amalgamates with the copper, spreads over it, and to a great extent lessens its efficiency; and as the copper

Fig. 549.

b

must be red hot to expel the mercury, much loss of copper as well as mercury is the result. To obviate this defect, the copper is connected above the zinc and left open at bottom; as, for example, a thin sheet of copper, of dimensions accord ing with the size of the cells in the battery, is cut thus: This cop per is bent in the middle at b, the ends a a dip into the cells, while

C

a

a

is bent over to connect with the zinc plate of the neighboring cell, thus:

The zinc plates are placed between the bent copper a a. The following diagram of a battery of several pairs of plates will illustrate these observations:

The zinc and copper are connected together by a binding screw. To construct this battery the zinc plates are put in first, being made to slide in grooves, cut in the sides of the trough, the plates standing in the centre of their respective cells: the copper plates are put in, and the copper bands marked c are made fast to the zines by binding screws, care being taken that the parts where they are connected are clean and bright, and that the copper and zinc touch nowhere else. A battery of nine pairs of plates can be fitted up and made ready for action in ten minutes.

In fitting up batteries of this sort, we are aware that sometimes great care is taken that the partitions in the trough be perfectly water-tight, and also formed of some non-conducting material, such as glass, or of wood, either pitched or saturated with some non-conducting substance; but we have found in practice that these precautions are not required, the principal thing to attend to being, that the metals should not be allowed to touch, except in their proper connections.

DEFECTS OF COMMON ACID BATTERIES.-Although we have spoken thus favorably of the principles upon which Wollaston's battery is constructed, still as a philosophical instrument it is far from being perfect: hence the many modifications of it which have been recommended. Indeed, electro-chemists, since the time of Volta, have been endeavoring to invent an instrument free from the defects which attach to Wollaston's--one capable of giving, at

the same time, a constant and powerful current, abundant in quantity, and of great intensity. The success and results of these endeavors are so closely connected with the art of electro-metallurgy, and the knowledge of them is so essential to a successful prosecution of the art, that we must not be sparing in our descriptive details. In operating with a Wollaston's battery, or any other arrange. ment composed of similar elements, such as zinc, sulphuric or muriatic acid, and copper, silver or platinum, it will be found that the current of electricity obtained diminishes in quantity and strength in proportion to the time of action. This is the result of various

causes:

1st. The hydrogen which is evolved at the surface of the negative metal in the battery, which we shall say is copper, adheres with considerable force to the surface of the metal, and consequently obstructs its superficial influence, so that the quantity of electricity which the surface of the two metals is calculated to give is much

lessened.

2d. After the battery is in action a short time, a portion of the sulphate or chloride of zinc, formed in the battery by the solution of the zinc, becomes reduced upon the surface of the copper. This reduction is supposed to be owing to the electrolyzation of the zine solution by the passage of electricity, but it is, more probably, caused by a galvanic action upon the copper plate and the solution in the battery. It generally begins at the lower edge of the copper plate, and spreads upwards. This weakens the electric current, both by inducing a galvanic action between the zinc and the copper, upon which it is deposited, and by its tendency to send a current of electricity in an opposite direction to the main current, thereby neutralizing to a great extent the original power of the circle.

3d. When copper is used it becomes gradually covered over with a thin, black, slimy coating of oxide and other impurities, which materially affects the regularity and strength of the current: this is a source of considerable annoyance in working, and necessitates a regular cleaning of the coppers, which should be done immediately on being taken out of the battery, by brushing with a hard hair brush in water; but when the battery has been long in action, this mode of cleaning is insufficient: the plates will then require to be rubbed over with a little dilute nitric acid, and then washed. If the black coating be allowed to dry upon the coppers, they must then be dipped into strong nitric acid till their surfaces are acted upon; or they may be moistened with a little urine, then brought to a dull red in the fire, and immediately plunged into water; but in both cases there is a loss of copper. A small quantity of the black matter, upon being tested, gave oxide of copper, with a trace of iron, antimony, and lead, which are the general impurities of sheet copper.

Max, Duke of Leuchtenberg,* has giving the following results

* Progress of General Science, vol. ii.