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. 21.19If, 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.
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 passing. 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 If the cylinder be withdrawn to a distance from K, it (the cylinder) will be found to show no signs of electrification.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 no20 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.
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 brass 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.21
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 principal 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 passing to E by different paths. Corresponding to the six lines falling on P from K, six others pass to E from the lower surfaces. In fig. 27 where K is just entering the pail, two lines only pass from K to E through the dielectric; the remaining ten fall on P, and ten others starting from the distant parts of P pass 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 pass 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
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of some interest to inquire whence is the source from which the energy of this apparently unlimited supply is drawn;22 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.
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 quantity 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 town23 of Ley-den where it was invented. It consists of a glass jar coated inside and out to a certain height with tinfoil, having a brass rod terminat-ing in a knob passed 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 knob with the charged plate of the electrophorus or by connecting the inner coating to one knob of an electrical machine and the outer coating to the other knob.
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 knob.
When the knob of the discharger is sufficiently close to the knob of the jar, a bright spark will be observed between the knobs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the knobs.
Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob 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 pass.
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.
