units to a potential V, whose energy of discharge is measured by QV,' we really mean that there is on the body a charge + Q, on the walls or other surrounding surfaces an equal and opposite charge Q, and that in the strained dielectric resides energy to the amount of

QV.

Dielectrics are bodies in which this electrostatic strain can be maintained; while conductors are those in which no such strain can be kept up, and in which therefore lines of force and fields of force cannot exist. In reality, bodies are not capable of sharp division into these two classes, since the yielding to electrostatic stress is only a matter of time. But in general we make use of bodies at the two extremes of the series; so that the foregoing definitions will serve very well for usual cases.

It is part of the above view to say that tubes of force are always terminated against surfaces so charged that at the two ends of each tube there are equal and opposite charges + 9 and - q respectively (see § 17).

It is also part of it to say that conductors act as screens; for lines of force cannot penetrate a conductor, and hence if an electrostatic field exist on each side of the screen, these two fields are independent of one another.

If lines of force were material, the phenomena of the electrostatic field would suggest that they act as stretched elastic threads, which moreover repel one another. Thus the two sides of a field are urged towards one another as if by tension of the lines of force connecting them.

CHAPTER XI.

THE PHENOMENA OF ELECTRIC CURRENTS. BATTERY CELLS AND BATTERIES.

§ 1. Introductory.-In Chapters IV.-X. we have dealt mainly with the phenomena connected with the actions between charged conductors, and with the strains of the intervening medium. We have, in fact, considered mainly electrostatic fields of force, or fields in which our test-charge of electricity is urged along lines of force.

We did, indeed, examine to some extent the results of discharge; but in general the readjustment of electrical equilibrium was so rapid that it was not easy to investigate the phenomena accompanying discharge. With such machines as the Holtz we might, however, have done so; but not so conveniently as with the help of other apparatus to be described in the present chapter.

We shall find that, during discharge, new classes of phenomena arise. In particular there are observed chemical, and heat, phenomena; and a new field of force, viz. a magnetic field (or a field in which a magnetic pole is urged), springs into existence.

In our next division of the subject we enter into the consideration of the phenomena accompanying electric discharge. As a rule we shall have very small AV.s as compared with those hitherto employed, but very large quantities of electricity, and an even and continuous flow. Where this is not the case, atten

tion will be drawn to the fact.

The above remarks will indicate that the popular terms 'statical electricity' and 'current electricity' must not be understood in their literal sense of two kinds of electricity; but must be considered to refer to two classes of phenomena that require different conditions of AV and of quantity for their investigation.

§ 2. Galvani's Experiment.-As stated in the preface, we shall not enter into any historical account of this or of any portion of our subject. But we must mention that experiment of Galvani's which, perhaps more than any other, started Volta on his very fruitful line of inquiry.

In the figure, ZC is a compound bar of zinc and copper, as indicated by the two letters employed. The zinc end is put into contact with the lumbar nerves of a frog's hinder quarters. It is found that whenever the copper end touches the muscles of the legs there is a sudden convulsion of the limb. This experiment

was first performed in 1786. Galvani considered that the metals acted merely as conductors to complete the circuit of 'animal electricity,' and followed out this idea in further investigations. As we do not intend to discuss in the present Course the relations of electricity to physiology, we shall not say more as to Galvani's views.

Volta, on the other hand, fixed his attention on the metals, and considered that they by their contact caused the current which the nerves and muscles of the frog's leg conducted. We shall have more to say as to his views, and as to modern modifications of them. For the present we will merely follow his experi

ments and the explanations that he gave; using, however, some terms, such as 'potential,' which he did not use.

§ 3. Volta's Experiments and Views. For clearness we will first state Volta's views, and will then give some of the experiments by which he supported them. The student can see how far each statement is supported or unsupported by the experiments given.

Volta's views. (1) When two heterogeneous substances are in contact they are found to be at different potentials.

(2) As a rule any ▲ V.s between liquid and liquid, or metal and liquid, are negligible as compared with the AV.s between metals and metals.

(3) Metals can be arranged in a certain series (such as zinc, tin, lead, iron, copper, silver, platinum, graphite), with respect to which the following facts hold: that any metal in contact with another occurring later in the list will have a higher potential than this latter; and that in a series of several metals in contact the ▲ V between the first and last is the same as it would be were these metals directly in contact. Thus, if we make the potential of graphite zero, and find the potentials of all the other metals with respect to this body when in contact with it, then we can calculate the AV between any pair in contact by simple subtraction of their potentials with respect to graphite.

(4) This law, however, does not hold good with respect to a series in contact when composed partly of metals and partly of liquids.

Thus, whereas in the series copper|zinc|gold copper there is no A V between the terminal metals, because it is the same as if the initial and final metals (viz. copper copper) were directly in contact, yet in the series copper|zinc|dilute acid copper there may be, and as a matter of fact there is, a AV between the terminal metals.

Experiments illustrating Volta's views.-In these experiments Volta employed the condensing gold-leaf electroscope, whose principle is explained in Chapter VI. § 14.

But far more satisfactory results can be obtained by use of any form of quadrant electrometer (say the Elliott' form of Chapter X. § 33), in which one pair of quadrants are to earth; such an instrument indicates quantitative results with an accuracy sufficient for lecture experiments.

(i.) The figure represents a simple experiment performed with the con

densing gold-leaf electroscope and a compound bar composed of a piece of zinc and a piece of copper soldered end to end.

We may use the quadrant electrometer, putting one pair of quadrants to earth, and connecting the other pair with an insulated terminal.

If the zinc be held in the hand, while we touch the lower condensing plate of the gold-leaf electroscope (or terminal of the electrometer) with the copper, it will be found that the condensing plate (or insulated pair of quadrants) is now at a potential.

(For use of electroscope and electrometer see Chapter X.)

M

N

FIG. i.

In this experiment it is assumed as sufficiently proved by various convergent pieces of evidence that the zinc, held in the moistened hand, is practically at the zero-V of the earth; and that the contact of the copper with the brass of the electroscope gives no ▲ V.

Hence, it is argued, the electroscope or electrometer indicates the ▲ V due to the contact of the zinc and copper alone, and shows that zinc is + to

copper.

(ii.) An electroscope is provided with an upper condensing plate made of zinc. This is, as usual, provided on its under surface with an insulating layer of lac varnish. Both plates are as usual carefully discharged by means of a Bunsen's flame, until no movement of the leaves is observed on raising or lowering the upper plate.