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For brevity we sometimes write it E. M.F. particular case it is obviously the result of the difference of potential, and proportional to it. Just as in waterpipes a difference of level produces a pressure, and the pressure produces a flow so soon as the tap is turned on, so difference of potential produces electromotive-force, and electromotive-force sets up a current so soon as a circuit is completed for the electricity to flow through. Electromotive-force, therefore, may often be conveniently expressed as a difference of potential, and vice versâ; but the student must not forget the distinction.

156. Volta's Laws.-Volta showed (Art. 71) that the difference of potential between two metals in contact depended merely on what metals they were, not on their size, nor on the amount of surface in contact. He also showed that when a number of metals touch one another the difference of potential between the first and last of the row is the same as if they touched one another directly. A quantitative illustration from the researches of Ayrton and Perry was given in Art. 72. But the case of a series of cells is different from that of a mere row of metals, for, as we have seen, when two metals are immersed in a conducting liquid they are thereby equalised, or nearly equalised, in potential. Hence, if in the row of cells the zincs and coppers are all arranged in one order, so that all of them set up electromotive-forces in the same direction, the total electromotive-force of the series will be equal to the electromotive-force of one cell multiplied by the number of cells.

157. Hitherto we have spoken only of zinc and copper as the materials for a battery; but batteries may be made of any two metals. That battery will have the

matter is moved by a magnet we speak rightly of magnetic force; when electricity moves matter we may speak of electric force. But E.M.F. is quite a different thing, not "force" at all, for it acts not on matter but on electricity, and tends to move it.

K

greatest electromotive-force, or be the most "intense," in which those materials are used which give the greatest difference of potentials on contact, or which are widest apart on the "contact-series " given in Art. 72. Zinc and copper are very convenient in this respect; and zinc and silver would be better but for the expense. For more powerful batteries a zinc-platinum or a zinccarbon combination is preferable.

electromotive-force

158. Resistance.-The same does not, however, always produce a current of the same strength. The strength of the current depends not only on the force tending to drive the electricity round the circuit, but also on the resistance which it has to encounter and overcome in its flow. If the cells be partly choked with sand or sawdust (as is sometimes done in socalled "Sawdust Batteries" to prevent spilling), or, if the wire provided to complete the circuit be very long or very thin, the action will be partly stopped, and the current will be weaker, although the E.M.F. may be unchanged. The analogy of the water-pipes will again help us. The pressure which forces the water through pipes depends upon the difference of level between the cistern from which the water flows and the tap to which it flows; but the amount of water that runs through will depend not on the pressure alone, but on the resistance it meets with; for, if the pipe be a very thin one, or choked with sand or sawdust, the water will only run slowly through.

Now the metals in general conduct well: their resistance is small; but metal wires must not be too thin or too long, or they will resist too much, and permit only a feeble current to pass through them. The liquids in the battery do not conduct nearly so well as the metals, and different liquids have different resistances. Pure water will hardly conduct at all, and is for the feeble electricity of the voltaic battery almost a perfect insulator, though for the high-potential electricity of the

frictional machines it is, as we have seen, a fair conductor. Salt and saltpetre dissolved in water are good conductors, and so are dilute acids, though strong sulphuric acid is a bad conductor. The resistance of the liquid in the cells may be reduced, if desired, by using larger plates of metal and putting them nearer together. Gases are bad conductors; hence the bubbles of hydrogen gas which are given off at the copper plate during the action of the cell, and which stick to the surface of the copper plate, increase the internal resistance of the cell by diminishing the effective surface of the plates.

LESSON XIV.-Chemical Actions in the Cell.

159. The production of a current of electricity by a voltaic cell is always accompanied by chemical actions in the cell. One of the metals at least must be readily oxidisable, and the liquid must be one capable of acting on the metal. As a matter of fact, it is found that zinc and the other metals which stand at the electropositive end of the contact-series (see Art. 72) are oxidisable; whilst the electronegative substances-copper, silver, gold, platinum, and graphite-are less oxidisable, and the last three resist the action of every single acid. There is no proof that their electrical behaviour is due to their chemical behaviour; nor is their chemical behaviour due to their electrical. Probably both result from a common cause. (See Article 422 (bis), and also p. 71.)

160. A piece of quite pure zinc when dipped alone into dilute sulphuric acid is not attacked by the liquid. But the ordinary commercial zinc is not pure, and when plunged into dilute sulphuric acid dissolves away, a large quantity of bubbles of hydrogen gas being given off from the surface of the metal. Sulphuric acid is a complex substance, in which every molecule is made up of a group of atoms,- -2 of Hydrogen, I of Sulphur, and 4 of

Oxygen; or, in symbols, H2SO4. The chemical reaction by which the zinc enters into combination with the radical of the acid, turning out the hydrogen, is expressed in the following equation:

Zn +

Zinc

H2SO4

=

Zn SO4 + H2

and Sulphuric Acid produce Sulphate of Zinc and Hydrogen. The sulphate of zinc produced in this reaction remains in solution in the liquid.

Now, when a plate of pure zinc and a plate of some less-easily oxidisable metal—copper or platinum, or, best of all, carbon (the hard carbon from the gas retorts)— are put side by side into the cell containing acid, no appreciable chemical action takes place until the circuit is completed by joining the two plates with a wire, or by making them touch one another. Directly the circuit is completed a current flows and the chemical actions begin, the zinc dissolving in the acid, and the acid giving up its hydrogen in streams of bubbles. But it will be noticed that these bubbles of hydrogen are evolved not at the zinc plate, nor yet throughout the liquid, but at the surface of the copper plate (or the carbon plate if carbon is employed). This apparent transfer of the hydrogen gas through the liquid from the surface of the zinc plate to the surface of the copper plate where it appears is very remarkable. The ingenious theory framed by Grotthuss to account for it, is explained in Lesson XXXVIII. on Electro-Chemistry.

These chemical actions go on as long as the current passes. The quantity of zinc used up in each cell is proportional to the amount of electricity which flows round the circuit while the battery is at work; or, in other words, is proportional to the strength of the current. The quantity of hydrogen gas evolved is also proportional to the amount of zinc consumed, and also to the strength of the current. After the acid has thus dissolved zinc in it, it will no longer act as a corrosive

solvent; it has been "killed," as workmen say, for it has been turned into sulphate of zinc. The battery will cease to act, therefore, either when the zinc has all dissolved away, or when the acid has become exhausted, that is to say, when it is all turned into sulphate of zinc. Stout zinc plates will last a long time, but the acids require to be renewed frequently, the spent liquor being emptied out.

161. Local Action.-When the circuit is not closed the current cannot flow, and there should be no chemical action so long as the battery is producing no current. The impure zinc of commerce, however, does not remain quiescent in the acid, but is continually dissolving and giving off hydrogen bubbles. This local action, as it is termed, is explained in the following manner :— The impurities in the zinc consist of particles of iron, arsenic, and other metals. Suppose a particle of iron to be on the surface anywhere and in contact with the acid. It will behave like the copper plate of a battery towards the zinc particles in its neighbourhood, for a local difference of potential will be set up at the point where there is metallic contact, causing a local current to run from the particles of zinc through the acid to the particle of iron, and so there will be a constant wasting of the zinc, both when the battery circuit is closed and when it is open.

162. Amalgamation of Zinc.—We see now why a piece of ordinary commercial zinc is attacked on being placed in acid. There is local action set up all over its surface in consequence of the metallic impurities in it. To do away with this local action, and abolish the wasting of the zinc while the battery is at rest, it is usual to amalgamate the surface of the zinc plates with mercury. The surface to be amalgamated should be cleaned by dipping into acid, and then a few drops of mercury should be poured over the surface and rubbed into it with a bit of linen rag tied to a stick. The mercury unites with the zinc at the surface, forming a

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