brevity we sometimes write it E.M.F. In this particular case it is obviously the result of the difference of potential, and proportional to it. Just as in water-pipes 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. The unit in which electromotive-force is measured is termed the volt (see Art. 354). The terms pressure and voltage are sometimes used for difference of potential or electromotive-force.

170 Volta's Laws. Volta showed (Art. 79) that the difference of potential between two metals in contact (in air) 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 touched 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. 80. But the case of a series of cells is different from that of a mere row of metals in contact. If in the row of cells the zines 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.

Hitherto we have spoken only of zinc and copper as the materials for a cell; but cells may be made of any two metals. The effective electromotive-force of a cell depends on the difference between the two. If zinc was

moves matter we may speak of electric force. But E.M.F. is quite a dif ferent thing, not "force" at all, for it acts not on matter but on electricity, and tends to move it.

used for both metals in a cell it would give no current, for each plate would be trying to dissolve and to throw a current across to the other with equal tendency. That cell will have the greatest electromotive-force or be the most "intense," in which those materials are used which have the greatest difference in their tendency to combine chemically with the acid, or which are widest apart on the "contact-series " given in Art. 80. Zinc and copper are convenient in this respect; and zinc and silver would be better but for the expense. For more powerful batteries a zinc-platinum or a zinc-carbon combination is preferable. That plate or piece of metal in a cell by which the current enters the liquid is called the anode ; it is that plate which dissolves away. The plate or piece of metal by which the current leaves the cell is called the kathode; it is not dissolved, and in some cases receives a deposit on its surface.

171. Resistance. The same electromotive - force does not, however, always produce a current of the same strength. The amount of 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 so-called "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 waterpipes 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 cell 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

172. Chemical Actions. 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 oxidizable, 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. 80) are oxidizable; whilst the electronegative substances copper, silver, gold, platinum, and graphite are less oxidizable, 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 that their chemical behaviour is due to their electrical.

Probably both result from a common cause (see Art. 80, and also 489). 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, 1 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 +

HSO4

=

ZnSO

+ H2

Zine 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 oxidizable metal- - copper or platinum, or, best of all, carbon (the hard carbon from 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 XLVII. 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 current. The quantity of hydrogen gas evolved is also proportional to the amount of zinc consumed, and also to 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.

173. 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 or parasitic current to run from the particles of zinc through the acid to the particle of iron, and so there will be a continual wasting of the zinc, both when the battery circuit is closed and when it is open.

174. Amalgamation of Zinc. We see now why a piece of ordinary commerical zinc is attacked on being placed in acid. There is local action set up all over its