The effects of points is illustrated in fig. 9; when a charged body is provided with a point as here shown, the current accumulates
at the point to such a high degree of density that it passes off into the air, and if a lighted candle be held in front of the point, the
flame will be visibly blown aside.9
Fig. 10 shows an electric windmill or experimental device for illustrating the escape of electricity from points. It consists of a vane of several pointed wires bent at the tips in the same direction, radiating from a center which rests upon a pivot. When mounted upon
the conductor of an electrostatic machine, the vane rotates in a direction opposite that of the points. The movement of the vane is due to the repulsion of the electrified air particles near the points and the electricity on the points themselves. The motion of the air is called electric wind. This device is also called electric flyer, and electric whirl.
Figs. 5 to 8.--Illustrating the distribution of the charge on conductors of various shapes.
"Free" and "Bound" Electricity.--These terms may be defined as follows:
The expression free electricity relates to the ordinary state of electricity upon a charged conductor, not in the presence of a charge
of the opposite kind. A free charge will flow away to the earth if a conducting path be provided.
A charge of electricity upon a conductor is said to be bound, when it is attracted by the presence of a neighboring charge of the opposite kind.10
Conductors and Insulators.--The term conductors is applied to those bodies which readily allow electricity to flow through them, in distinction from insulators or so-called non-conductors, which practically allow no flow of electricity.
10
Strictly speaking, there is no substance which will prevent the passage of electricity, hence, the term non-conductors, though extensively used, is not correct.
Fig. 9.--Experiment to illustrate the effect of pointed conductors.
Fig. 10.--Electric windmill which operates by the reaction due to the escape of the electric charge from the points. Electroscopes.--These are instruments for detecting whether a body be electrified or not, and indicating also whether the electrification be positive or negative. The earliest electroscope devised consisted of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle will serve equally well. Another form of electroscope is the pith ball pendulum, shown in figs. 2 and 3. When an electrified body is held near the electroscope it is attracted or repelled thus indicating the presence and nature of the charge.11
Gold Leaf Electroscope.--This form of electroscope, which is very sensitive, was invented by Bennet. Its operation depends on the fact that like charges repel each other.
Fig. 11.--Gold leaf electroscope; it consists of two strips of gold foil suspended from a brass rod within a glass jar. Used to detect the presence and sign of an electric charge.
The gold leaf electroscope as shown in fig. 11, is conveniently made by suspending the two narrow strips of gold leaf within a wide mouthed glass jar, which both serves to protect them from draughts of air and to support them from contact with the ground. A piece of varnished glass tube is pushed through the cork, which should be varnished with shellac or with paraffin wax. Through this passes a stiff brass wire, the lower end of which is bent at a right angle to receive the two strips of gold leaf, while the upper end is attached to a flat plate of metal, or may be furnished with a brass knob.
When kept dry and free from dust it will indicate excessively small quantities of electricity. A rubbed glass rod, even while two or three feet from the instrument, will cause the leaves to repel one another. If the knob be brushed with only a small12 camel's hair brush, the slight friction produces a perceptible effect. With this instrument all kinds of friction can be shown to produce electrification.
The gold leaf electroscope can be further used to indicate the kind of electricity on an excited body. Thus, if a piece of brown paper
be 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