on the surfaces of metallic electrified bodies was well known and shown by Cavallo in his book published in 1777. The experiment quoted is called

THE ELECTRIC WELL.

66 Place upon an electric stool a metal quart mug, or some other conducting body nearly of the same form and dimension; then tie a short cork-ball electrometer at the end of a silk thread proceeding from the ceiling of the room, or from any other proper support, so that the electroscope may be suspended within the mug, and no part of it may be above the mouth; this done, electrify the mug by giving it a spark with an excited electric, or otherwise, and you will see that the electroscope, whilst it remains in that insulated situation, even if it be made to touch the sides of the mug, is not attracted by it, nor does it acquire any electricity; but if, whilst it stands suspended within the mug, a conductor, standing out of the mug, be made to communicate with or only presented to it, then the electroscope acquires an electricity contrary to that of the mug, and a quantity of it which is proportionable to the body with which it has been made to communicate; and it is then immediately attracted by the mug. Cavallo explains the cause in his own quaint language, and his theory is in accordance with that taught in these days, only the technical names are changed; thus, in modern style, the fact would be explained by stating that "polarity cannot be set up when opposing actions are at work in different directions, as in the inside of an insulated metallic vessel." Cavallo says, "The reason why, in this experiment, the electroscope contracts no electricity whilst suspended entirely within the cavity of the mug is because the electricity of the mug acts upon the electroscope on all sides, and this has no opportunity of parting with its fluid when the mug is electrified positively, nor of receiving any when the mug is electrified negatively. But, as soon as any conductor communicates with it, the electroscope becomes immediately possessed of the electricity contrary to that of the mug; for, if the mug be electrified positively, the fluid belonging to the electroscope will be repelled to that body which communicates with it, and which, being out

of the mug, cannot be affected by its electricity; and if the mug is electrified negatively, it will attract the fluid of the electroscope, which actually receives an additional quantity of it from that conducting body with which it com

A, quart mug insulated, and containing the electroscope inside; B, the threads raised above the edge of the vessel, or, still better, touched with an insulated brass rod extending into the air. In A, opposing forces, + and +, oppose in different directions In B, polarity can be set up; because the inside is +, the electroscope -, and the extremity of the rod in the air +.

"The electroscope, therefore, becoming always possessed of a contrary electricity, must necessarily be attracted.

"If, by raising the silk thread a little, part of the electroscope, i.e., of its linen threads, are lifted just above the mouth of the mug, the balls will be immediately attracted; for then, by the action of the electricity of the mug, it will acquire a contrary electricity by giving to or receiving the electric fluid from the air above the cavity of the mug.

"It has been supposed by some that the electroscope in the above experiment (or any other small insulated body), hanging in the cavity of an electrified vessel, or the like, is not attracted by the sides of the vessel because the attraction of electricity, being as the squares of the distances inversely, cannot affect the electroscope one way more than another; it being demonstrable that if to every point of a spherical concave surface equal centripetal forces are directed, decreasing as the squares of the distances from those points, a small body situated anywhere within that surface would remain there without being attracted one way more than another. But to this it may be replied that the demonstration of the above-mentioned proposition, if it is applicable to spherical or cylindrical concave surfaces, cannot, however, be applied to every kind of irregular cavities, with which, if they exceed not a certain size, the above experiment succeeds as well as with the cylindrical cavity of the mug."

Cavallo proceeds to give what he considers to be the proper theory, which in the main is right; but, as before observed, the explanation is simplified by stating that, as polarity cannot be set up inside a vessel, so a charge cannot be maintained.

*Newton's "Principia," Book I., prop. lxx.

ELECTRICAL INDUCTION.

In studying the phenomena of light and heat, it will be necessarily observed that these forces have a radiant power. A heated body may be brought towards another which is not heated, and impart to it a certain amount of its warmth; the latter gains what the former loses: the vibratory power set up in the heated body is supposed to be conveyed by the undulations of the ether to the body which is not heated, and setting up therein similar vibrations; the result is that heat is produced in a cold substance by the approach of a heated body, which loses its vibrating energy in warming the other.

Loss of power, independent of any conducting power of damp air, curious to say, is not observed when an electrified body is gradually brought towards another which is not electrified; and yet the electrical quiescence of the latter is disturbed, and may give rise to large quantities of electricity, as in Holtz's electrical machine (Fig. 202); the effect thus obtained is called "induced electricity."

The fact is well shown by using a cylindrical conductor, the two halves of which can be separated with their respective insulating glass columns. On the underside of the conductor small rings or hooks may be inserted for the convenience of attaching pairs of gilt pith-balls, which should be as light as possible.

A, the electrified ball approached to the conductor, B C, made in two halves to separate at D; each half to have one suspended pith-ball at D, so that, when joined together, they form a pair of balis as in the ordinary electroscope; also each to have a pair at the extremities B and C.

Directly the charged ball A has approached sufficiently near to the conductor BC, the pith-balls show by their mutual repulsion that its electrical quiescence is disturbed, and that, in fact, if the ball has been charged with positive or + electricity, it will cause negative or electricity to become apparent at B, whilst positive or + electricity will be found at C. The pith-balls hanging at D will hardly be disturbed, if at all, showing that there is a neutral point, like the centre of a bar magnet, where the forces are balanced. When the disturbing cause A is removed, the separated electricities rush together again, the electrical equilibrium of the cylinder is restored, and the pith-balls no longer repel each other.

No advantage, therefore, so far as the production of a permanent charge of electricity, has been obtained in the above experiment, which, it must be remembered, is performed with a conductor of electricity. If, however, the experiment is repeated, and, whilst the conductor is under induction from the ball A, the two halves are separated, then it will be found that each half is permanently electrified.

A, the electrified ball; B, the half of the cylinder, separated from the other, and showing a charge of negative or-electricity; c, the other half, showing positive or + electricity; D, the single balls, suspended from B and C, attract each other, as they represent the opposite electricities, + and -.

The separation of the halves of the conductor whilst under induction has prevented the opposite forces reuniting; the pith-balls remain deflected on each half, and the single balls, suspended at the place where the two halves are separated, incline towards each other, because dissimilar electricities attract. The equality of the electrical disturbance is again beautifully shown by bringing the halves together, when the electrical excitation set up entirely ceases, as the two opposite forces exactly neutralize each other.

The experiment may be once more repeated, and the two halves separated whilst under induction. If a stick of excited wax is approached to the half of the cylinder marked B, minus, the pith-balls are deflected still further from each other; but when the same stick of excited sealing-wax is brought towards C, or plus electricity, the balls drop down.

In the first case, the increased deflection shows that the electricity on B is negative, because the wax is negative, and exalts the previous charge. In the second, the diminished deflection and falling down of the balls show that the electricity on C is positive, as it is neutralized for the time being by the influence of the negatively electrified wax.

Two electroscopes, one placed in connexion with each half of the conductor, may be substituted for the pith-balls, and are, perhaps, more certain and truthful in their indications; moreover, they are more delicate, and would show a smaller amount of electrical disturbance.

These experiments demonstrate that, in conductors, polarity, i.e., the separation of the electricities, the production of opposite properties in opposite direction, may be set up by induction, but is not maintained; and this is, in fact, as contended by Faraday, the essential difference between conductors and nonconductors: in the former polarity is not maintained; in the latter, as we shall

now see, polarity, being set up, is maintained, or it would be impossible to charge a Leyden jar.

When a plate of glass is held against the ball attached to the prime conductor of an electrical machine, and a pith-ball, suspended on a glass support, is approached towards it, the ball is energetically attracted towards the glass; and yet the latter, being called a non-conductor, ought not to have permitted the electricity to have apparently travelled, like heat, through its sub

stance.

B

FIG. 214.

A, one side of the glass plate, which may be one foot square, and is held against the ball of the electrified conductor; B, the ball suspended on the glass stand, and attracted to the other side of the glass plate.

The electricity does not travel through the glass plate, but, like the brass conductor (Fig. 212), is thrown into an electro-polar state, the one side touching the conductor being positive, and the other side, to which the pith-ball is attracted, being negative; a very slight charge is thus conferred upon the glass plate, which will not rise higher until one side is put in conducting communication with the ground. The small charge, however, is retained when the glass is removed, and thus the polarity is shown to be maintained by non-conductors, constituting the essential difference between them and conductors of electricity.

The sheet of glass cannot be charged properly unless both surfaces are