Thus, in practice, from 3 to 6 ampere-hours may be stored for each kilogramme of lead.

Experience shows that, except for very slight losses, which, of course, increase with the time which elapses between the charge and the discharge, accumulators in being discharged restore almost all the electricity which had been given to them during the charge. It must, however, be remarked that the quantity of energy restored is always necessarily less, since the charge is effected at a higher potential than the discharge. If E is the electromotive force, R the resistance of the battery, and C the strength of the current during the charge, E', R', and

FIG. 146.

161. Electro-Capillary Phenomena. The polarisation of a surface by changing the condition of the surface modifies those properties which, like the surface-tension, are a function of this condition.

Suppose two masses of mercury separated by acidulated water (Fig. 146), one, A, contained in a funnel with so fine a point that the mercury is in equilibrium owing to the action of the meniscus, while the other, B, forms a large surface at the bottom of the vessel C. If a platinum wire, a, dipping in the top vessel, is connected with the negative terminal of a battery, and an insulated platinum wire, ẞ, dipping with its bared end in the mercury in the lower vessel, is connected with a point on a wire joining the terminals, the potential of the mercury A, becomes less than that of the mercury B, and the difference of potentials becomes greater and greater as the resistance of the part of the battery-circuit, between the points to which the wires a and B are attached, is increased. As this takes place, the surface-tension of the mercury in the drawn-out point increases, and the mercury recedes farther and farther from the point, until the difference of potentials has reached a value of about 0.9 volt.

If we mark the position of the mercury when the two masses are directly connected by joining the conducting wires aß, and are therefore at the same potential, the pressure which must be exerted on the surface of the mercury, to bring the meniscus to the mark, might serve as a measure of the surface-tension, and at the same time of the difference of potential.

Lippmann's capillary electrometer (Fig. 147) is based on this principle. The mercury A is contained in a long tube, terminated

FIG. 147.

by a finely drawn out point. By means of a caoutchouc bag the desired pressure can be exerted at the top of the tube. The pressure can be measured by means of the manometer H, and the position of the index can be read off by the microscope M. From the pressure the difference of potential is deduced by means of a table constructed once for all.

This apparatus is very sensitive, and serves to measure differences of potential from o to 0.9 volts. But it is of special use in what are called zero methods. The difference of potential to be measured is introduced into the circuit with the electrometer, and along with it a known difference of potential which can be varied at will, like that obtained between two points of a wire, AB (Fig. 148), traversed by a constant current (§§ 109, 115); the position of one of these points is varied until the mercury comes back to its mark; the two mercuries are then at the same potential, and

P

the electromotive force is equal to the fall of potential between the two points, A and B.

The apparatus in Fig. 146 can be used to make a curious experiment on reversibility. If the height of the mercury in the funnel is just so small that there is no spontaneous flow by the point this commences as soon as the two wires, aß, are connected,

1

FIG. 148.

and at the same time the circuit is traversed by a current proceeding through the wire from the mercury B to the mercury A.

CHAPTER XVI.

MAGNETISM-GENERAL PHENOMENA.

162. Natural and Artificial Magnets.-The name magnet or loadstone is applied to certain specimens of natural oxide of iron (Fe3O4) which possess the property of attracting iron filings. All points of the loadstone do not possess this property to the same degree; when such a stone is rolled in the filings, they adhere chiefly to certain parts.

By simple friction, and without itself losing any of its properties, the stone can impart to steel the property of attracting iron. Steel magnets are also called artificial magnets, in opposition to the former, which are called natural magnets. Experiment shows, further, that the properties of the two kinds of magnets are identical, and accordingly, from their simpler and more regular form artificial magnets are the only ones used. They are ordinarily employed in the form of needles (Fig. 151) or bars (Fig. 149). 163. Poles of a Magnet.—When a bar is placed in iron filings, these adhere chiefly to the ends, as if the property of attracting iron were specially concentrated there (Fig. 149). These ends are

FIG. 149.

called the poles of the magnet. The idea was long ago conceived that the bar consists of inert matter possessing two centres of action near its ends. Such centres of action would be poles, and owe their properties to acting masses called magnetic masses concentrated at them.

The centres of action are really disseminated over the two ends of the bar; this is seen very clearly from the manner in which the filings attach themselves to the bar, and in a still more striking

manner from the way in which they arrange themselves in the following experiment.

A thin sheet of glass or cardboard is placed above a bar magnet, and iron filings are sifted uniformly over it. On giving the plate slight taps, the filings group themselves in regular curves (Fig. 150), which start from one point of the surface, and terminate at the symmetrically situated point on the other side of the centre. This experiment, the bearing of which we shall afterwards explain, shows, first of all, the existence of a field of force about the magnet. It demonstrates, moreover, this important fact, that magnetic action is transmitted without alteration through any substance which is

not itself magnetic. This property is common to magnetism and gravity. Exception must be made for iron and a few other substances, which when they are sufficiently thick can screen the action of a magnet.

164. Distinction between the Poles-Action of the Earth.-As regards the action of a magnet on iron filings, there is no distinction between one pole and the other; the two poles are nevertheless not identical.

A second property which every magnetised bar possesses is that, when suspended horizontally, it takes up a fixed direction in space, which in this part of the world is not very far from that of north and south. The same end always turns to the north; this