tricity in the cell, since we could use a primary current to effect the original electrolysis, and then could recover a current after a greater or less interval of time. So understood, the term is a misnomer. We use up some of the electrical energy of the primary current, and get equivalent chemical potential-energy stored up in the secondary-cell; later, when we complete the circuit of this cell, we lose this chemical-potential-energy and get again the electrical energy of a current. But we did not store up electricity; a

condenser does this, not a secondary-cell.

Analogy. We might use up some wind-energy to turn a fan, and this might wind up a weight from a lower to a higher level. We might, later on, let the weight again descend, and so obtain a wind driven in the reverse direction by the fan. We should have stored up mechanicalpotential-energy, not wind.

The problem of inventing a good secondary-cell is of a twofold nature.

(i.) How can we retain a large amount of the ions in contact with the electrodes, ready to drive a reverse current?

(ii.) How can we prevent action occurring while the cell is lying idle with open circuit?

In our next two sections we shall describe one form of cell devised to satisfy these two conditions.

§ 13. Planté's Secondary-Cell.-In the original form of Plante's cell there are initially two lead plates immersed in dilute H2SO4. I. Formation of the cell.-A somewhat lengthy process is needed in the first instance in order to get the cell ready to act as a battery.

(i.) A current is passed through it; this resulting in hydrogen being given off at the kathode, while the surface of the anode is oxidised into the condition of PbO2.

(ii.) The current is reversed, and is continued until the PbO2 is all reduced to spongy lead, while the other plate is in its turn per-oxidised.

(iii.) This process of sending currents in alternate directions is repeated until the lead has been acted upon to some depth. Thus, the plate that served last as anode is left coated deeply with PbO2, that which served last as kathode is deeply coated with spongy lead. This process is called 'Formation of the cell,' and it

is said to be left charged'; this is represented in fig. i. Subsequent chargings will need only a single passage of the current.

II. Discharging the cell.

In our discussion of the cell we will at present consider only the degree of oxidisation of the lead, and will only touch on the part played by the dilute acid. It is clear that if we have PbO, the form

ation of PbSO, from this (PbO + H2SO, PbSO, + H2O) can be regarded as a secondary action, and has no direct bearing on the electrolytical theory of the cell.

If we connect the terminals of the cell, a current is set up in a reverse direction to the current that was last passed through the cell in its formation. One atom of oxygen from each molecule of PbO2 passes back to the other plate, giving PbO on each plate, instead of PbO2 on the one and Pb on the other. This action from the one plate across the liquid to the other plate takes place only step by step with the current; and the current will continue as long as there is PbO, left. When the cell has 'run down,' each plate will be coated with PbO (or, by action with the dilute acid, with Pb.SO4, which is equivalent to PbO as regards degree of oxidisation).

III. For the cell to be recharged, a current must be again passed through the cell. If we consider that there is PbO on both sides, the action is simple; by an electrolytic passage of H, to

2

the kathode, and of O to the anode, we get again Pb on the former and PbO2 on the latter.

If there be Pb.SO, on both plates we may represent the action as follows. [See fig. (iii.)]

For convenience we have written Pb.SO4 + H2O (there is plenty of available water in the dilute acid) in the two equivalent forms H2SO4.PbO and OPb. H2SO4.

(i.) That the electrolytic action of charging' the cells is the same, whether there be PbO or Pb.SO, on the two plates of the exhausted cell.

(ii.) That the recharging of the cell is practically a transference of O from the kathode to the anode.

IV. Local action, and waste of energy, &c.-It may naturally occur to the reader that there seems no reason why the whole action should not take place between the PbO, and the lead plate upon which it rests, instead of between the PbO, and the other lead plate that is separated from it by the dilute acid; so that after a short period of lying idle with open circuit, the cell would have 'run down' and be useless.

No doubt there is some such action at first. But it seems that the insoluble layer of PbSO4, which is thus formed between the PbO, and the lead plate on which it rests, hinders further local action; while the surface turned towards the other lead plate, being a free surface, remains always more open and porous. This appears to be a very important part played by the insoluble PbSo

As regards the energy wasted in the charging of the cell, this will be more fully discussed in Chapter XV. We will here only

remark that we waste more energy in heat when the back E.M.F. of the secondary-cell is small as compared with that of the primary battery, or when the charging current is large.

It is of interest to remark that by charging cells arranged 'in parallel,' and then coupling them ‘end-on,' we can obtain a secondary battery of as great an E.M.F. as we please; one that could drive a current back through the primary battery.

§14. Faure's Accumulator.-In order to obviate the necessity for the lengthy and energy-wasting process of 'formation,' Faure devised the following important modifications in Planté's cell.

The two lead plates were coated with minium, this being Pb2O3, or PbO.PbO2. One passage of the current then sufficed, by the electrolytic setting free of H, at the one plate and of O at the other, to convert the one layer into spongy lead, and the other into lead peroxide.

Since, however, Pb,O, requires but O in order to per-oxidise it, while it requires 3H, to reduce it, it is clear that we must have three times the amount of minium on the anode (where the electrolytic oxygen is set free) as on the kathode (where the hydrogen is set free). Thus the electrolysis of 3H2O will reduce Pb2O, on the kathode, and will per-oxidise 3(Pb,O) on the anode.

In all other respects the theory of the 'Faure's accumulator,' as it is called, is the same as that of the Planté's cell. The question of energy will be discussed more fully later on.

CHAPTER XIII.

OHM'S LAW.

§ 1. General Ideas as to the Scope of Ohm's Law.-Up to the present point we have, in our treatment of the battery-cell, of the current driven by it, and of the circuit through which the current flows, used terms in a vague and qualitative, rather than in an exact and quantitative sense.

But in the present chapter we propose to discuss at some length the conditions which determine the magnitude of an electric current in any particular case; and to state and explain the law which makes the calculation of the current a matter of simple arithmetic. The law referred to is that known as Ohm's law; it was enunciated by Dr. G. S. Ohm in the year 1827. This law-which must be accepted as confirmed by countless direct and indirect experiments and refuted by none-states in the first place that when the circuit of a cell is completed we are concerned with three quantities only; and in the second place that these three quantities are connected by a very simple relation, viz., that given in § 2.

I. Electromotive force.-Each battery-cell possesses a certain power of driving a current, which is directly proportional to, and can be measured by, the ▲ V that is found to exist between the poles when the circuit is broken. Since that which moves matter is called force, so by analogy that which moves 'electricity' was called 'electromotive force.'

The reader must remember that this term is an inexact one, as the 'electromotive force' is not a force at all in the scientific sense defined in Chapter II. § 4. To prevent confusion we shall henceforth use the letters E. M.F. instead ; so that the actual word force will never be used except in the strict sense of Chapter II. § 4.