267.]

GROVE'S NITRIC ACID BATTERY.

535

intense voltaic action. Platinum is the least liable of the metals to chemical action, whilst amongst the metals that admit of being easily wrought, zinc is the one which is most readily attacked by acids; consequently the opposition of platinum to zinc furnishes a most effective voltaic combination; whilst nitric acid absorbs with ease the hydrogen liberated on the platinum, and thus forms water and nitrous acid, which remain in solution in the undecomposed acid; the resulting liquid constitutes one of the most perfect of liquid conductors. If HNO,O in the following diagram represent nitric acid, and H,SO, diluted sulphuric acid, Pt the platinum plate, and Zn the zinc one, the molecular arrangement will be indicated before the action by the position of the brackets above, and after the action by the position of those below.

PtH NO, O | H. SO, H2SO Zn

With a battery of ten such cells, 5 inches (12 centimetres) high and 2 inches (6 centimetres) wide, a large number of brilliant experiments may be performed, but four or five cells are generally sufficient for most purposes of electro-chemical decomposition. If oil of vitriol be mixed with the nitric acid in the porous cells in the proportion of about equal measures, a current is thus obtained, the strength of which is more uniform than when nitric acid only is used (Callan).

With a view to economy, Bunsen substitutes for the platinum plates in Grove's battery, cylinders of carbon, prepared by heating together a mixture of powdered coke and caking coal, or powdered coke moistened with a strong solution of sugar. A firm coherent cake is thus obtained. Cylinders made of this material auswer well while new; but being porous, the carbon absorbs the nitric acid, which corrodes and impairs the surface of contact with the zinc. A better material is the hard carbon from the gas retorts, but it is difficult to shape it into the form of plates. Poggendorff has employed plates either of sheet iron or cast iron instead of either platinum or carbon; in strong nitric acid the iron is totally unacted on; but if the acid become diluted till it has a density of 135, or less, it is liable to act upon the metal with uncontrollable violence. No combination possesses the intense energy, in union with convenience of working and comparative durability, in the same degree as that proposed by Grove. It is necessary, however, to place the nitric acid battery so that the nitrous fumes (which are copiously evolved during its action, especially alter the battery has been in use for some time) shall pass at once into the open air; as they would otherwise seriously incommode the operator.

(267) Smee's Battery.-The other mode of obviating the counteracting agency of hydrogen upon the negative plate of the battery is less perfect, and is of a mechanical nature. It was first practically applied by Smee in the construction of the voltaic battery. Hydrogen adheres to smooth surfaces of platinum and

536

SMEE'S BATTERY.

[267.

other metals with considerable force, but it passes off with ease from their asperities and edges: by multiplying their points and irregularities, as, for example, by the deposition of metal on the surface in a pulverulent form, the escape of the gas is much facilitated. Smee employs as the negative or conducting plate in his battery, a plate of silver, the face of which has been roughened by the deposition of finely divided platinum upon its surface; each side of the silver plate being exposed to a plate of zinc well amalgamated, and of equal extent, which acts as the positive plate. This pair of plates is excited by means of diluted sulphuric acid.

Fig. 221 represents a battery constructed upon Smee's principle: a thin platinized silver plate is supported in a light frame of wood, as shown detached

at s; to the upper part of this frame a binding-screw, in metallic connexion with the silver, is fastened, for the purpose of connecting the plate with the zine plates of the adjoining cell, by means of a strip of sheet copper bent as at c; on either side of the silver plate a sheet of amalgamated zinc, z, z, is supported by the clamp shown at b; the zinc plates are prevented from contact with the silver plate by means of the wooden frame, and they are connected with the silver of the adjacent cell by a second binding-screw in the clamp shown at b; the separate plates are attached to a wooden frame, and being counterpoised by weights, as indicated in the figure, can be lowered into the trough of acid when wanted for use, or can be withdrawn from it when the experiment is over. The trough is divided into separate cells or compartments for each pair of plates, by glass partitions rendered water-tight by means of a resinous cement.

(268) Resistances to the Voltaic Current.-The current which circulates in any given circuit is not dependent solely upon the energy of the chemical action which is exerted between the generating metal and the exciting liquid. The current experiences a resistance from the very conductors by which it is transmitted; just as in the transmission of work, the intervention of the pivots and levers which are required for its conveyance intro

268.]

RESISTANCES TO THE VOLTAIC CURRENT.

537

duces additional friction, which requires to be overcome, and which thus diminish the efficient power of the machine.

The resistance to the voltaic current may be considered as of two kinds-first, that resistance which arises from the exciting liquid employed in the voltaic cell itself; and secondly, that which arises from the conducting wire and apparatus exterior to the voltaic cell. In a large number of cases the resistance offered by the exciting liquid is by much the most considerable, and it is inseparable from the combination; whilst the second source of resistance, or that which is exterior to the cell, can be increased or diminished at pleasure, and by the employment of very short and thick wires for connecting the plates, can be virtually removed altogether or annihilated. It will be advisable to consider first the resistance produced by the liquid in the active cell itself.

If plates of equal size be taken, the resistance occasioned by the liquid increases directly as the distance between the plates; the longer the column of imperfectly conducting matter which the current has to traverse, the greater is the difficulty which it will experience. If two plates be immersed in acid at the distance of an inch (25 millimetres) asunder, they will produce 10 times the effect that they would occasion at the distance of 10 inches or 250mm. from each other. On the other hand, the larger the area of the plates that are immersed, the less is the resistance. For example, if a pair of plates, 25mm. broad and 250mm. long, be immersed in acid to the depth of 25' 5mm. only, the current produced will only be equal to one-tenth of that which would be obtained by immersing each plate for its whole depth of 250mm. in the liquid. The resistance of the liquid is therefore directly as the distance between the plates, and inversely as the surface of the plates exposed to its action. A pair of plates exposing each a square centimetre of surface, immersed in acid at a distance of I centimetre apart, will consequently produce an effect equal to that which would be obtained from a pair of plates which each exposed a surface of 12 square centimetres to the action of the liquid, if they were 12 centimetres apart.

A case somewhat analogous is offered when water is transmitted through pipes. The greater the length of the pipe, the more considerable will be the friction and the consequent resis tance to the passage of the liquid; whilst the larger the area of the pipe the more readily will the water escape. An aperture which exposes a sectional area of two square centimetres will, cæteris paribus, allow twice as much water to escape from it in

538

RESISTANCES TO THE VOLTAIC CURRENT.

[268.

a given time as an aperture of which the superficial area is but a single square centimetre.

If the two plates are of unequal size, but are immersed parallel to each other, they may, for most practical purposes, be calculated as each exposing a surface equal to the mean surface of the two. Other circumstances independent of the extent of surface exposed by the plates, and the distance between them, materially influence the resistance of different liquids to the current. Any cause that favours chemical action between the active metal and the liquid, or which diminishes the chemical attraction by which the elements of the liquid are united, such as elevation of temperature, diminishes the resistance of the liquid. In most cases an increase in the concentration of the solution, provided its strength be not so great as to render deposition of crystals liable to occur, diminishes the resistance (278). The current likewise experiences a specific resistance in each liquid which depends upon the chemical attraction with which its particles are united together.

Similar, but distinct resistance, though to a less extent, is offered by the metallic part of the circuit. However great its conductivity may be, it always offers some obstruction to the current. The longer the wire employed, the greater is the resistance experienced by the current in traversing it. The resistance of each metal, like that of each liquid, is specific. Copper and silver, for instance, when wires of equal thickness and length are compared, offer far less resistance to a current of given amount than less perfect conductors, such as iron and lead. Experiment has demonstrated that with metallic conductors the same law holds good as with liquids-viz., that the resistance is directly as the length of the wire, and inversely as the area of its section. In cylindrical wires this sectional area will of course vary as the square of the diameter of the wire; for instance, a wire of 1 in thickness will for equal lengths offer four times. the resistance of a wire 2mm. thick. If wires of the same metal, and of equal lengths, be compared, the resistance will vary inversely as the weights of the wires.

In the experiment with cupric sulphate (fig. 214), the metal is deposited in greatest quantity where the current is most readily transmitted-viz., in those points which are nearest to the zinc, and where the resistance offered by the liquid, which here forms the thinnest layer, is consequently the least.

A rod of zinc supported within a cylinder of copper forms a convenient arrangement of the generating and conducting plates, because, when such a rod is placed in the axis of the cylinder,

269.]

SIMPLE AND COMPOUND CIRCUITS.

539

the action is evenly distributed over the whole surface of the

copper.

(269) Difference between a Simple and a Compound Circuit.The observations hitherto made have referred to cases in which only a single pair of metals is employed. It will be necessary now to consider in what way the results are modified by the employment of several pairs of plates. It has already been stated when speaking of the electricity developed by friction, that when a large supply of electricity is needed, it may be obtained with equal effect either from a single Leyden jar which exposes a large extent of coated surface, or from a number of smaller jars which together expose the same amount of coated surface, all the inner surfaces of the small jars being in metallic communication with each other, but insulated from the outer coatings, all of which likewise are connected by some good conducting material (235). A similar result is also obtained in voltaic arrangements. Provided that the plates expose the same extent of surface and be kept at an equal distance apart, it matters not whether they be immersed in a single vessel of liquid, or whether they be cut up into strips and be immersed in pairs in separate vessels of the same liquid. The only requisite is that all the zinc plates shall be connected together by stout metallic wires, and that all the platinum plates shall be similarly connected by other wires. No action will occur until metallic communication between one of the platinum and one of the zinc plates is effected by means of a conducting wire: and then the whole force of the united plates will traverse the connecting wire.

These results may be exhibited to the eye in the form of battery in which the hydrogen evolved from each platinum plate admits of being collected -a contrivance proposed by Daniell, which he called a dissected battery. Fig 222 shows the manner of mounting one of these cells. When in use the cells are charged with the diluted sulphuric acid, and a small graduated jar, H, also filled with the diluted acid, is inverted in each of the cells over the platinum plate, P, in such a manner as to receive the hydrogen which is disengaged during the ope

FIG. 222.

ration. The plates of such a battery can easily be connected