To repeat the experiment in modified form, let an electroscope be placed beneath a bird cage or wire netting, as in fig. 15.

Let charged rods or other powerfully charged bodies be brought near the electroscope outside the cage. The leaves will be found to remain undisturbed.

[Ill.u.s.tration: FIG. 15.--The electric screen. A screen of wire gauze surrounding a delicate electrical instrument will protect it from external electrostatic induction.]

=Electrification by Induction.=--An insulated conductor, charged with either kind of electricity, acts on bodies in a neutral state placed near it in a manner a.n.a.logous to that of the action of a magnet on soft iron; that is, it decomposes the neutral electricity, attracting the opposite and repelling the like kind of electricity. The action thus exerted is said to take place by _influence_ or _induction_.

The phenomenon of electrification by induction may be demonstrated by the following experiment:

In fig. 16, let the ebonite rod be electrified by friction and slowly brought toward the k.n.o.b of the gold leaf electroscope.

The leaves will be seen to diverge, even though the rod does not approach to within a foot of the electroscope.

[Ill.u.s.tration: FIG. 16.--Experiment to ill.u.s.trate electrostatic induction.

The leaves will diverge, even though the charged ebonite rod does not approach to within a foot of the electroscope.]

This experiment shows that the mere _influence_ which an electric charge exerts upon a conductor placed in its vicinity is able to produce electrification in that conductor. This method of producing electrification is called _electrostatic induction_.

As soon as the charged rod is removed the leaves will collapse, indicating that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.

=Nature of the Induced Charge.=--This is shown by the experiment ill.u.s.trated in fig. 17.

Let a metal ball A be charged by rubbing it with a charged rod, and let it then be brought near an insulated metal cylinder B which is provided with pith b.a.l.l.s on strips of paper C, D, E, as shown.

The divergence of C and E will show that the ends of B have received electrical charges because of the presence of A, while the failure of D to diverge will show that the middle of B is uncharged. Further, the rod which charged A will be found to repel C but to attract E.

[Ill.u.s.tration: FIG. 17.--Experiment ill.u.s.trating the nature of an induced charge. The apparatus consists of a metal ball and cylinder, both mounted on insulated stands, pith b.a.l.l.s being placed on the cylinder at points C, D, and E.]

From these experiments, the conclusion is that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of opposite sign.

=The Electrophorus.=--This is a simple and ingenious instrument, invented by Volta in 1775 for the purpose of procuring, by the principle of induction, _an unlimited number of charges of electricity from one single charge_.

It consists of two parts, as shown in fig. 19, a round cake of resinous material B, cast in a metal dish or "sole" about one foot in diameter, and a round disc A, of slightly smaller diameter made of metal or of wood covered with tinfoil, and provided with a gla.s.s handle. Sh.e.l.lac, or sealing wax, or a mixture of resin sh.e.l.lac and Venice turpentine, may be used to make the cake.

[Ill.u.s.tration: FIGS. 18 and 19.--The electrophorus and method of using.

Charge B; place A in contact with B, and touch A (fig. 18). The disc is now charged by _induction_ and will yield a spark when touched by the hand, as in fig. 19.]

To use the electrophorus, the resinous cake B must be first beaten or rubbed with fur or a woolen cloth, the disc A is then placed on the cake, touched with the finger and then lifted by the handle. The disc will now be found to be charged and will yield a spark when touched with the hand, as in fig. 19.

The "cover" may be replaced, touched, and once more removed, and will thus yield any number of sparks, the original charge on the resinous plate meanwhile remaining practically as strong as before.

The theory of the electrophorus is very simple, provided the student has clearly grasped the principle of induction.

[Ill.u.s.tration: FIGS. 20 to 23.--Ill.u.s.trating "how the electrophorus works."]

When the resinous cake is first beaten with the cat's skin its surface is negatively electrified, as indicated in fig. 20. Again, when the metal disc is placed down upon it, it rests really only on three or four points of the surface, and may be regarded as an insulated conductor in the presence of an electrified body. The negative electrification of the cake therefore acts inductively on the metallic disc or "cover," attracting a positive charge to its under side, and repelling a negative charge to its upper surface, as shown in fig. 21.

If, now, the cover be touched for an instant with the finger, the negative charge of the upper surface (which is upon the upper surface being repelled by the negative charge on the cake) will be neutralized by electricity flowing in from the earth through the hand and body of the experimenter. The attracted positive charge will, however remain being bound as it were by its attraction towards the negative charge on the cake.

[Ill.u.s.tration: FIG. 24.--Lines of force of a charged sphere and a conductor under induction. The negative electrification on the end _a_ of the cylinder indicates that a certain number of lines end there, while the positive electrification on the end _b_ similarly indicates that an _equal_ number of lines set out from that end. It is one of the fundamental properties of a conductor that it yields instantly to the smallest electric force, and that no electric force can be permanently maintained within the substance of a conductor in which no current is pa.s.sing. There can, therefore, be no electrostatic strain and no lines of force within the material of a conductor where the electric field has become steady. Hence the lines starting from _b_ are entirely distinct from those ending at _a_. The two sets are equal in number because no charge has been given to the cylinder, either positive or negative, and therefore the sum of all the positive electrifications (or lines starting from _b_) must be equal to the sum of all the negative electrifications (or the lines ending at _a_). In all nine lines have been drawn at each end of the cylinder, leaving the thirteen lines emanating from the sphere which do not run on to the cylinder. If the cylinder be withdrawn to a distance from K, it (the cylinder) will be found to show no signs of electrification.]

Fig. 22 shows the result after the cover has been touched. If, finally, the cover be lifted by its handle, the remaining positive charge will no longer be "bound" on the lower surface by attraction, but will distribute itself on both sides of the cover, and may be used to give a spark. It is clear that no part of the original charge has been consumed in the process, which may be repeated as often as desired. As a matter of fact, the charge on the cake slowly dissipates--especially if the air be damp.

Hence it is needful sometimes to renew the original charge by again beating the cake with the cat's skin.

[Ill.u.s.tration: FIG. 25.--Faraday's ice-pail experiment. An ice-pail P connected with the gold leaves of an electroscope C, is placed on an insulating stand S. A charged conductor K, carried by a silk thread, is lowered into the pail, and finally touches it at the bottom. While it is being lowered the leaves of the electroscope diverge farther and farther, until K is well within the pail, after which they diverge no more, even when K touches the pail or is afterwards withdrawn by the insulating thread. After withdrawal, K is found to be completely discharged.]

The labor of touching the cover with the finger at each operation may be saved by having a pin of bra.s.s or a strip of tinfoil projecting from the metallic "sole" on to the top of the cake, so that it touches the plate each time, and thus neutralizes the negative charge by allowing electricity to flow in from the earth.

[Ill.u.s.tration: FIGS. 26 to 29.--Explanation of Faraday's ice pail experiment. For simplicity the electroscope, insulating stand and silk thread have been omitted. Only the three princ.i.p.al conductors K, P, and the earth E are shown. In fig. 26 the ball K is sufficiently close to P to act inductively on it; six lines are shown as falling on P, and the other six as pa.s.sing to E by different paths. Corresponding to the six lines falling on P from K, six others pa.s.s to E from the lower surfaces. In fig.

27 where K is just entering the pail, two lines only pa.s.s from K to E through the dielectric; the remaining ten fall on P, and ten others starting from the distant parts of P pa.s.s to E. In fig. 28, K is so far within P that none of its lines can reach E through the dielectric; they all fall on P and from the outside of P an equal number start and pa.s.s through the dielectric to E. It is evident that in this position K can be moved about within P, without affecting the outside distribution in the slightest, and that even when K touches P as shown in fig. 29, and when, therefore, all lines between them disappear, the lines in the dielectric outside remain just as they are in fig. 28. K is now completely discharged, since lines no longer emanate from it, hence it can be removed by the silk cord without disturbing the electrification of P. If K be again charged and introduced into P it will be again discharged, for the fact that P is already charged will have no effect on the final result, provided when K touches P it is well _under cover_.]

Since the electricity thus yielded by the electrophorus is not obtained at the expense of any part of the original charge, it is a matter of some interest to inquire whence is the source from which the energy of this apparently unlimited supply is drawn; for it cannot be called into existence without the expenditure of some other form of energy. The fact is, _more work is done in lifting the cover when it is charged_ with the positive electricity than when it is not charged; for when charged, there is the force of the electric attraction to be overcome as well as the force of gravity; this excess force is the real origin of the energy stored up in the separate charges.

[Ill.u.s.tration: FIGS. 30 and 31.--The Leyden jar and discharger. Its discovery is attributed to the attempt of Musschenbrock and his pupil Cuneus to collect the supposed electric "fluid" in a bottle half filled with water. The bottle was held in the hand and was provided with a nail to lead the "fluid" down through the cork to the water from the electric machine. The invention of the Leyden jar is also claimed by Kleist, Bishop of Pomerania.]

=Condensers; Leyden Jar.=--A _condenser_ is an apparatus for condensing a large quant.i.ty of electricity on a comparatively small surface. The form may vary considerably, but in all cases it _consists essentially of two insulated conductors, separated by an insulator and the working depends on the action of induction_.

A form of condenser generally used in making experiments on static electricity is the Leyden jar, so named from the town of Leyden where it was invented. It consists of a gla.s.s jar coated inside and out to a certain height with tinfoil, having a bra.s.s rod terminating in a k.n.o.b pa.s.sed through a wooden stopper, and connected to the inner coat by a loose chain, as shown in fig. 30.

The jar may be charged by repeatedly touching the k.n.o.b with the charged plate of the electrophorus or by connecting the inner coating to one k.n.o.b of an electrical machine and the outer coating to the other k.n.o.b.

The discharge of a condenser is effected by connecting the plates having an opposite charge. This may be done by use of a wire or a discharger, as shown in fig. 31; the connection is made between the outer coat and the k.n.o.b.

When the k.n.o.b of the discharger is sufficiently close to the k.n.o.b of the jar, a bright spark will be observed between the k.n.o.bs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the k.n.o.bs.

Let a charged jar be placed on a gla.s.s plate so as to insulate the outer coat. Let the k.n.o.b be touched with the finger. No appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger. Again no appreciable discharge will appear. But if the inner and outer coatings be connected with the discharger, a powerful spark will pa.s.s.

=Electric Machines.=--Various machines have been devised for producing electric charges such as have been described. The ordinary "static" or electric machine, is nothing but a continuously acting electrophorus.

Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of the stationary plate E, are pasted paper sectors, beneath which are strips of tinfoil AB and CD called _inductors_.

In front of E is a revolving gla.s.s plate carrying discs _l_, _m_, _n_, _o_, _p_ and _q_, called _carriers_.

To the inductors _AB_ and _CD_ are fastened metal arms _t_ and _u_, which bring _B_ and _C_ into electrical contact with the discs _l_, _m_, _n_, _o_, _p_ and _q_, when these discs pa.s.s beneath the tinsel brushes carried by _t_ and _u_.

A stationary metallic rod _rs_ carries at its ends stationary brushes as well as sharp pointed metallic combs.

The two k.n.o.bs _R_ and _S_ have their capacity increased by the Leyden jars _L_ and _L'_.

[Ill.u.s.tration: FIG. 32.--The Toepler-Holtz electric machine.]

[Ill.u.s.tration: FIG. 33.--Principle of Toepler-Holtz electric machine.]

=Action of the Toepler-Holtz Machine.=--The action of the machine described above is best understood from the diagram of fig. 33. Suppose that a small + charge is originally placed on the inductor _CD_. Induction takes place in the metallic system consisting of the discs _l_ and _o_ and the rod _rs_, _l_ becoming negatively charged and _o_ positively charged.

As the plate carrying _l_, _m_, _n_, _o_, _p_, _q_ rotates in the direction of the arrow the negative charge on _l_ is carried over to the position _m_, where a part of it pa.s.ses over to the inductor _AB_, thus charging it negatively.

When _l_ reaches the position _n_ the remainder of its charge, being repelled by the negative electricity which is now on _AB_, pa.s.ses over into the Leyden jar _L_.

When _l_ reaches the position _o_ it again becomes charged by induction, this time positively, and more strongly than at first, since now the negative charge on _AB_, as well as the positive charge on _CD_, is acting inductively upon the rod _rs_.