720

The Voltaic Battery.

and the galvanic element or cell is but a particular instance of the general class. The molecular disturbance may be produced by heat as well as by chemical action, and gives rise to electric currents exactly similar in character to those of the galvanic cell.

958. When several cells are united so that their effects may be combined, a galvanic or voltaic battery is formed.* The figure (fig. 266) shows one of the earliest combinations of this sort. It is called a trough battery; and consists simply of a trough,

N' P, divided by glass, or slate, or glazed porcelain, into a series of water-tight compartments. A number of galvanic pairs of metal plates are affixed to a wooden bar, N P, so that they can, when desired, be lowered simultaneously into the trough. The plate, N, is zinc, and forms the negative pole or terminal to which the conducting wire is fastened: the next pairs-copper and zinc-are each joined together by a band of copper; and so at the extreme end, P, a single plate of copper is left, to which the other wire or terminal is attached. The cells are filled with salt water, or water acidulated by of its volume of vitriol; and on immersing the metal plates in these a very powerful current flows through the wire connecting the terminals.

By uniting many such troughs very extraordinary effects may be produced, as the sequel will show. Sir Humphry Davy, at the Royal Institution in London, had a battery composed of two thousand pairs, with which he obtained astounding effects, and with which he made his principal discoveries.

* Named respectively after two Italian philosophers, GALVANI and VOLTA, who discovered the facts.

Many improvements on this original form of battery have since been devised, to describe all of which is beyond the scope of the present treatise. We shall, therefore, explain briefly the action of the principal and typical forms which have proved themselves most efficient for experimental, as well as telegraphic and other practical, purposes.

P

Z

959. Daniell's battery and its varieties.-Professor Daniell invented the form of cell represented in fig. 267, which for general purposes is still one of the very best forms. It consists of a copper dish or cylinder, C, within which is placed a porous dish of unglazed porcelain, which latter contains the negative element, a rod of castzinc, Z. Dilute acid acts upon the zinc in the porous dish, and a strong solution of blue vitriol, or the sulphate of copper, is placed between the porous cell and the copper element; some undissolved crystals being also placed in the solution to keep it strong or concentrated. The advantage of this compound arrangement is that the porous division-which might also be of bladder, or paper, or anything of this nature-prevents the passage of metallic particles between the copper and zinc elements, which takes place rapidly in the former arrangement where the two are placed face to face. Not only so, but there is an advantageous secondary action or effect of the current upon the sulphate of copper solution, by which the metallic copper is displaced from its union with the sulphuric acid and deposited bright and pure on the copper dish, thereby keeping its metallic surface clean and ready for constant action. These two qualities combine to make the battery in a remarkable degree constant and invariable in the strength of current which it evolves.

Fig. 267.

A combination of the Daniell principle with the trough arrangement explained above is now much employed for telegraphic purposes. Each of the water-tight compartments in the above trough (fig. 266) is simply divided into two by a porous diaphragm; the copper plate with blue vitriol solution goes into the one half and the zinc plate with dilute sulphuric acid into the other. Several other contrivances have been adopted for the purpose of dispensing with the porous separating plate altogether, and still keeping the liquids distinct. But the advantage of economy is counterbalanced by

722

Grove's and Bunsen's Batteries.

increased awkwardness in construction of the cells so as not to admit of ready inspection or repair.

The following modification of the Daniell cell has been found to work very well. A shallow glass dish, D, forms the outer vessel; it is half or third filled with fine clean sand. A ring of glass is pushed down so as to be about an inch under the sand surface. A strip of copper sheet, bent into a ring, is placed round the glass ring and forms the positive element or pole of the cell; while a rod or lump of cast-zinc, placed inside the glass ring so as to rest on the sand, forms the negative element. The action takes place through the sand, and the liquids can be replenished with great readiness.

Fig. 268.

Callaud's battery is pretty much the same in principle as this, only less capable of ready inspection. In the latter, the copper plate is placed at the bottom of the cell among sulphate of copper crystals; the whole is covered with a layer of sand, and the zinc plate laid on the top of this.

960. Grove's battery and Bunsen's battery are perhaps the most important of all on account of their superior current-power, or electromotive force.

Fig. 269 will give an idea of the Bunsen cell, which differs from Daniell's only in two particulars: first, a cylinder of coke or carbon, C, stands within the porous cell, taking the place of the copper in Daniell's, as the positive pole; and, second, this positive element is immersed in strong nitric acid instead of the cupric sulphate in the Daniell. The negative element, as in other batteries, is zinc, and the exciting liquid in the outer cell, dilute sulphuric acid. The Bunsen element is much more powerful than any of those we have

N

Fig. 269.

described, and is very extensively employed on the Continent. In this country a variety of the same type of element more commonly used is known as the Grove element. It differs from the Bunsen only in the substitution of a platinum plate for the carbon of the former; and, though more expensive at the outset, a battery of this form is more economical in the end, and more cleanly to work with, than the Bunsen.

The purpose which the nitric acid serves in both of these batteries is to remove the hydrogen gas, which is liberated at the positive element by a series of de

Recent Forms of the Voltaic Battery.

723

compositions and recompositions. It is found that when the liberated hydrogen is not removed, it reduces the power of the battery very rapidly, both by covering the positive plate with a nonconducting gaseous layer, and by an opposite electro-motive force which the hydrogen sets up. This secondary action is termed polarization of the plates of the battery, and in single fluid batteries, its effect is so great as to reduce their power very rapidly.

Iron

The Maynooth battery is a modification of the two last. immersed in strong nitric acid, or in a mixture of strong nitric and sulphuric acids, is not attacked as we should expect, and as it naturally is, when immersed in a weaker acid solution; not only so, but it is thrown into the passive state, as it is called, acquiring a sort of permanent unoxidisable skin over it, which enables it to be used as the negative element, instead of platinum or carbon, in the cells we have described.

To avoid the disagreeable and dangerous fumes of nitrous gas given off by all these three forms of battery, the bichromate of potash is proposed as a substitute for the nitric acid. It is mixed with five times its weight of vitriol, and ten times its weight of water, and the zinc is acted on by brine, or a saturated solution of common salt. But this battery is less reliable in its constancy, and its electro-motive force is less.

Marié Davy's sulphate of mercury battery is a recent ingenious modification of the Bunsen. In it, the carbon plate is surrounded with a thick solution or paste of the sulphate of mercury; while the zinc is acted on by brine or weak acid. The liberated gas in the positive cell displaces the mercury from its combination with the sulphuric acid, and metallic mercury is deposited on the carbon, speedily collecting in drops and by its weight falling to the bottom of the porous cell.

961. The Leclanché cell is a cheap form of battery which has become popular within a few years. In it the plate of carbon in the porous cell, is packed round with a mixture of coarse carbon and binoxide of manganese; while in the outer or zinc cell, a solution of chloride of ammonium or sal-ammoniac acts as the exciting liquid. This action causes the formation of chloride of zinc in the outer cell, and the liberated hydrogen reduces the manganese oxide to a lower oxide. When a long-continued current of no great power is required, as in short telegraphic circuits, or for electric bells, such a battery is said to be very suitable.

Such are the leading forms of the voltaic or galvanic battery

724

Oxidability of the Metals used.

which have been devised; but many others have been proposed and, to a greater or less extent, adopted. These do not require a special notice.

962. It may be stated as a general principle that any two metals having a difference in their degree of oxidability, or in their readiness to oxidize, may be used with a dilute acid to generate an electric current. The greater the degree of oxidability, the greater the electric current or electro-motive force of the couple. The order of oxidability for the principal metals is as follows :—

If we associate any two of these as a galvanic couple, we produce a current ; and the farther apart the metals are in the list, the stronger the current produced. The current is always considered to flow up the list, that is from the metal least acted on to the other. Thus zinciron in dilute acid will give a current from the iron towards the zinc ; zinc-copper, a stronger current; zinc-platinum, a stronger still; and zinc-carbon, the strongest current of all. This explains the superior power of the Bunsen and Grove arrangements of battery. No other combination in the list can, by the very nature of things, produce such a strong current when placed in dilute acid. It may be observed, however, that this order is by no means invariable for all acids; with a difference in the nature of the exciting liquid, considerable variations in the order are introduced.

Two liquids also, having different affinities for any metal, will give rise to a current when united by a single wire or plate of this metal. For example, if in a Grove cell, the zinc and sulphuric acid be replaced by a strong solution of caustic potash, and if the nitric acid and the potash be connected by a platinum wire, a current will flow from the nitric acid towards the potash.

963. In the manipulation of batteries the following points should be attended to :-(i.) The metals used for the poles or electrodes should be as pure as possible; this prevents the interference of minute local currents, which consume the plates unnecessarily and weaken the general current materially. Pure zinc is very expensive,