glass surface returns to the rubber in a neutral, or nearly neutral, condition. But the rubber was left charged

-.

Hence, if the

original separation of + or were to go on, then after some turns the rubber would become highly-charged, and the tendency to part with some of its charge to the glass would at some point in the process at last balance the tendency to excite the glass +ly. Things would come to a standstill.

electricity

But if we put the rubber to earth, we shall get the initial condition of the rubber maintained by a steady passage of to earth. Later on we shall treat of this matter from a potential point of view.

So, if the prime conductor be put to earth, we can use the rubber as a source of electricity. If the rubber and conductor

both be insulated, but they be nearly connected by conducting rods, when we turn the cylinder we shall see a series of sparks bridge over the gap left; the + charge of the conductor and the -charge of the rubber continually neutralising each other.

II. The common plate machine.—In this there is nothing essentially different from the cylinder machine. A glance at the figure will explain all.

There are generally two rubbers; and in this form of machine they cannot well be insulated if required; so the machine cannot be used as a source of both + and - electricity. Instead of glass, ebonite plates may be used, the rubbers being of amalgamated silk.

Note. With ebonite two precautions must be taken; (a) never to heat the plate, only to warm it gently; (b) after use to clean the plate with paraffin oil from which all water has been removed by the introduction of a lump or two of sodium into the bottle.

III. Winter's plate machine.—In this the rubber and the points of the prime conductor are more widely separated; and the prime conductor can therefore acquire a higher level (or potential) of charge without discharge over the glass to the rubber. The rubber can be insulated or not, as required. A curious feature is an addition to the prime conductor in the shape of a large ring of brass enclosed in baked wood. This ring

increases the 'capacity' (see Chapter V. $ 4 &c.) of the prime conductor.

The machine gives very long sparks for its size, as compared with an ordinary plate machine.

$ 19. Miscellaneous Experiments with the Electrical Machine.

Experiments. — (i.) Illustrating the action of points. Besides the simple experiment consisting in discharging the conductor either by presenting to it an earth-connected point, or by fixing the point on to it, experiments can be easily performed in which the rush of elec

trified air from the points is made use of. We may blow out a candle by presenting it to the point.

Or again, we may by means of the electric vare,' shown in the figure, make evident the repulsion between the charged particles of air and the points

whence they derive their charge. The air will stream one way, the vane rotate in a contrary sense.

(ii.) The insulating stool.—A person standing on a stool supported by glass legs can be electrified by his placing the hand on the prime conductor while the machine is worked. He can then 'give sparks' to other persons, light gas with sparks, &c.; and his hair will, if fine in quality, show a tendency to stand on end.

CHAPTER V.

INTRODUCTORY CHAPTER ON POTENTIAL.

(For further, see Chapter X.)

§ 1. Quantity of Electrification.-We have spoken of + and - electrification, or electricity, as something measurable and divisible, that is as a quantity. Formerly it was spoken of also as a fluid; a view that was fairly justified by certain observed phenomena, but not by others.

This conjunction of the words 'quantity' and 'fluid' leads us to consider in what respect there is, and in what respect there is not, a resemblance between electricity on the one hand, and such fluids as water and gases on the other.

(a) Points of resemblance.—Electricity can be measured as a quantity, as can water and gases.

In any system of vessels between which there is proper connection, there can be statical equilibrium of water only when all is at the same level, or of a gas only when all is at the same pressure. So on any electrical conductor there can be electrical equilibrium only when there is throughout the same electrical level or electrical potential.

In the former case the water or gas will move from places of higher level or of greater pressure to places of lower level or less pressure respectively, until the state of equilibrium is arrived at. So will there be a flow of electricity from places of higher to places of lower potential, or of — electricity in the opposite direction, until a uniform potential is arrived at.

Water, and gases, and electricity, are alike unalterable in quantity; only in the case of electricity we must remember that it is the algebraic sum of any + and quantities that is constant,

equal and opposite charges giving a zero sum.

Note. In the above we have neglected the compressibility of water, and the action of gravity on gases, as relatively insignificant.

F

(b) Points of difference.-Water and gases all possess mass; indeed, from a mechanical point of view they are merely masses endowed with certain mechanical properties, their chemical composition signifying nothing.

Electricity on the other hand has, most probably, no mass. There is, in the case of electricity, no gravitation-force, no mechanical inertia, no mechanical kinetic energy. In a word, electricity is not matter, while all fluids are.

If we are to call electricity 'a thing' at all, it must be justified by the precedent of calling energy a thing'; and we merely express by the term the unalterability of electricity as a quantity.

In the case of water and gases we have little or nothing answering to the duality of electricity; nothing like the attractions and repulsions between unlike and like charges respectively; nor is there anything like the variation of the intensities of these forces with the nature of the medium in which the charged bodies are situate (see Chapter X. § 25, end).

To return to the subject of our section.

Electricity is measured as a quantity; and the unit is that quantity which at unit distance (i.e. 1 cm.) from an equal quantity repels it with unit force, or with one dyne; the medium between the two quantities being assumed to be air.

§ 2. Electrical Level, or Electrical Potential.--In the consideration of the statical equilibrium of bodies of water, or the work to be got out of any water supply, we are concerned mainly with level, differences of level, and quantity of water.

We usually take 'the sea-level' as our arbitrary zero of level, and reckon the level of any body of water as so many feet + or; the feet being measured vertically up from, or down from, this arbitrary zero respectively.

Now in electricity we have, roughly analogous to the above, the question of electrical potential, difference of electrical potential, and quantity of electricity.

We take as a zero of level (whether it is quite arbitrary or not we do not here discuss) the electrical potential of the carth; and we reckon potentials as + or when above or below our zero respectively.

In gravity levels the test of the same level and different levels is