Page images

electricity will be likewise doubled. However, the extent of surface of the positive metal, within certain limits, is not of so much consequence, although too great a deficiency of this is attended with detriment.

(17.) One circumstance must be noticed, that every point of the negative offers a radiating point to the positive metal; for every point not so situated, is much less active, and sometimes even perfectly inactive. In different cases, this property is shown more or less strikingly; for if the hydrogen be removed in its nascent state, it will, under the combined action of its adhesion and elasticity, manifest itself at a great distance from the positive metal, and even quite without the sphere of its radiation, as is the case where the back of a piece of metal is active, while the front alone is opposite to the fluid. When very smooth metals are used, it will also pass to a great distance; but when a metal is prepared in the manner I have hereafter to point out, by platinum, the gas will only be given off from a small extent, though very violently, when touched by the point of a fine zinc wire.

(18.) A relation exists between the power, and the distance interposed between the electro-positive and negative metals; for the nearer these can be brought together, the greater the quantity of electricity developed; though the intensity is not influenced by the difference of arrangement.

(19.) The function of the acid solution has already been partially explained; for we have before mentioned that the water is decomposed, the hydrogen being evolved at the negative metal, and the oxygen combining with the zinc, to form oxide of zinc. The acid now comes into play, and in addition to its adding considerable conducting power to the solution, it removes the oxide to form the sulphate of zinc.

(20.) Whatever exciting fluid is employed to charge the battery, its efficacy depends upon the same principles; but the intensity varies with each variation in the foreign body placed in the water; thus dilute nitric acid, dilute sulphuric

acid, or a solution of salt, all impart different intensities to the battery; an increase, however, or diminution in the proportion of these, does not interfere in the intensity, though the quantity is materially altered; for if but ten drops of dilute sulphuric acid are placed in a gallon of water, the intensity would be the same as if a pint of acid were employed; but the quantity in one case would be infinitely less than in the other.

(21.) A galvanic battery exhibits two important properties, quantity and intensity. Quantity depends directly upon the size of the negative metal or strength of the solution, while intensity depends upon much more hidden causes. Quantity requires but one cell, and this has been referred to in all the preceding experiments, for we have seen that we must have two metals with an intervening fluid. These will remain inactive while they do not touch; but as soon as contact takes place, either in the exciting fluid, at a distance, or through a fluid of more easy decomposition than the exciting fluid of the battery, the action immediately commences. The contact may be made through a great length of wire, with the same result. In this case, however, if the wire be either long, of small diameter, or of a metal of no great conducting power, it will be seen that the hydrogen evolved from the negative metal will be materially lessened, showing that an obstacle is presented to the electric fluid; but if intensity be given to the fluid, then the hydrogen will be evolved as freely as before.

(22.) To obtain this intensity we must have recourse to a number of galvanic batteries, arranged as a series; that is, the zinc of one battery connected with the silver of the next, and this in regular continuation, leaving the extreme zinc and silver free. In this way a hundred batteries may be conjoined, but no more quantity obtained; for only the same amount of electricity passes as when one cell is used. Now, however, this same amount can pass through a much greater resistance, for it would seem as if at every alternation of the


battery, the electric fluid obtained a push to overcome any obstacle afforded to its passage, and this push is called its intensity.

To the beginner, these two properties are very difficult to understand, but perhaps a rough idea may be formed of them by comparing quantity to the piston imparting motion to a railway train, which moves readily with one engine on a level road; let the train meet an obstacle, as an inclined plane or a hill, two, three, or 100 engines may be required to move this same train over, and yet the piston which turns the wheels of the carriage would move no more times than if one engine had been employed.

(23.) There is no advantage, but even a loss, in using a battery with an intensity more than sufficient to overcome a resistance; whether produced by a fluid to be decomposed, or by any other means; for if ten cells arranged as a compound battery be sufficient to overcome the obstacle, the effect of sixty cells, arranged as six tens, would be six times as much as if a single ten were used, because they would then form a battery of six times the size, but of the same intensity as before; but if the whole were used as one compound series, the resulting decomposition would be considerably less than six times the quantity, and to use a battery with advantage this fact must be borne in mind. If again, the surfaces be increased before sufficient intensity be obtained, in like manner it will not add a proportionate amount of power.

(24.) A compound galvanic battery, or one of many cells, has the same quantity of electricity passing in each cell, and therefore the same quantity of zinc dissolved. On this account the fewer the cells that can be employed to overcome the obstacle, the greater will be the economy. It is obvious therefore, that as soon, as by increasing the series, sufficient intensity has been attained to overcome partially the resistance, quantity should be sought by increasing the surface; for when one cell, as a single series, requires one pound of zinc to do a given

amount of work, when that same work is done more quickly by twelve cells, twelve pounds are dissolved-one pound in each cell; and of whatever size the cells may be, still the result will be the same, for no more zinc will be dissolved.

(25.) The simplest form of compound battery is the Couronne des Tasses, which is composed of alternate slips of zinc and platinum soldered together; the zinc is to be placed in one glass, the platinum in the next; and the series, thus arranged, may be charged with dilute sulphuric acid; care must be taken that the metal of the alternate pairs do not touch in the fluid.

(26.) When intensity alone is required, a large number of small plates should be used, as in De Luc's column, which is constructed of pairs of plates of dissimilar metals, separated by paper. There are several methods by which it may be made; the most common of which is to place alternate discs of silvered paper on similar discs of zinc, taking care that the series (i. e. the relative position of zinc) has always the same direction. It may be also made of discs of silvered or gilt paper, the uncovered side having been first spread over with the black oxide of manganese and honey. However, care must be taken that the manganese be not exposed to the sun; as in that case it is rendered inert; and also that the silver or gilt paper be not covered with any varnish, as that which is usually sold in the shops; 500 to 1000 discs must be employed to make an efficient instrument.

(27.) The larger batteries, which were in use for a number of years, consisted generally of copper and zinc, arranged in different forms, according to the fancy of the operator. Thus, the copper of each cell surrounded the zinc, and both were united to fit into a porcelain trough, with eight, ten, or more cells. Here each cell is to be considered as a distinct battery, although the copper and zinc of the whole trough are united, an arrangement contrived to remove the series of batteries from the trough at one time.

(28.) In this compound battery, a porcelain diaphragm


separates each simple battery; but Dr. Hare discovered that a series of batteries might be placed in one vessel, provided that the metals of each battery did not touch in the fluid, and that neither the electro-positive metal afforded a radiatory power to the electro-negative metal, of any other but its own pair, nor that any electro-negative metal radiated in a similar manner to any electro-positive metal. This form of battery very little known in this country, and I believe but seldom used any where.


(29.) There is another form which was devised by Cruikshank, and which consists merely of square pieces of zinc and copper, soldered together, and fixed at regular intervals in a wooden trough; the zinc always being in one direction. In this battery the metals themselves divide the cells.

(30.) There are many other forms of compound batteries, which do not require particular mention, as the principles which have been already explained, affect them all.

(31.) Provided the metals be sufficient to carry the current, their thickness does not influence the quantity of electricity, that depending upon the surface exposed to the fluid; but if the metals be so thin that they cannot carry the electricity, a diminution in the quantity of the current produced will ensue, similar to that which arises from thin wires. For this rea

son, earthenware, coated with platinum was not found to answer for the negative plate of a Grove's battery, the platinum surface not being of sufficient thickness. Yet, however thin a metallic or good conducting surface be employed, the current will gradually traverse it; a property of no small importance for the electrotype.

(32.) As the metals are good conductors, and the metallic oxides non-conductors, it is important that the negative metal should expose a clean metallic surface, or else it will be perfectly inert; therefore, when the old forms of batteries are employed, the copper should be thoroughly cleansed from oxide, before the battery is put in action.

(33.) When the metal is thoroughly cleaned before it is

« PreviousContinue »