style="font-size:15px;"> The gold leaf electroscope can be further used to indicate the kind of electricity on an excited body. Thus, if a piece of brown paperbe rubbed with a piece of india rubber, the nature of the charge is determined as follows:
Fig. 12.--Distribution of electrification on a charged hollow sphere. If an insulated conductor d be inserted through the opening
in the sphere and brought in contact with the interior surface and afterwards carefully removed, it will be found, by testing with
the gold leaf electroscope, that it has received no charge. If touched to the outside, however, the conductor will receive part of the charge.
First charge the gold leaves of the electroscope by touching the knob with a glass rod rubbed on silk. The leaves diverge, being electrified with positive electrification. When they are thus charged the approach of a body which is positively electrified will cause them to diverge still13 more widely; while, on the approach of one negatively electrified, they will tend to close together. If now the brown paper be brought near the electroscope, the leaves will be seen to diverge more, proving the electrification of the paper to be of the same kind as that with which the electroscope is charged.
The gold leaf electroscope will also indicate roughly the amount of electricity on a body placed in contact with it, for the gold leaves open out more widely when the quantity of electricity thus imparted to them is greater.
Figs. 13 and 14.--Electrification produced by rubbing dissimilar bodies together and then separating them. If the insulated glass and leather discs A and B be rubbed together, but not separated, no signs of electrification can be detected; but if the discs be drawn apart a little distance the space between them is found to be an electric field, and as they separate farther and farther, electric forces will be found to exist in more and more of the surrounding space, the electrification being indicated by "lines of force." It should
be noted that work has to be done in separating the charged discs to overcome the attraction which tends to hold them together.
The stress indicated by the lines of force consists of a tension or pull in the direction of their length and a pressure or thrust at right angles to that direction.
Electric Screens.--That the charge on the outside of a conductor always distributes itself in such a way that there is no electric
force within the conductor was first proved experimentally by Faraday. He covered a large box with tin foil14 and went inside with
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the most delicate electroscopes obtainable. Faraday found that the outside of the box could be charged so strongly that long sparks
would fly from it without any electrical effects being observable anywhere inside the box.
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.
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 analogous to that of the action of a magnet on soft iron; that is, it decomposes the neutral electricity, attracting the opposite and repelling the15 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 knob 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.
Fig. 16.--Experiment to illustrate 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 pro-
duce 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 illustrated in fig. 17.16
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 balls 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.
Fig. 17.--Experiment illustrating the nature of an induced charge. The apparatus consists of a metal ball and cylinder, both mounted on insulated stands, pith balls 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.17
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 glass handle. Shellac, or sealing wax, or a mixture of resin shellac and Venice turpentine, may be used to make the cake.
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.
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The "cover" may be replaced, touched, and once more removed, and will thus yield any number of sparks, the original18 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.
Figs. 20 to 23.--Illustrating "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
