electrified. If two silk ribbons of the same kind be rubbed across each other, that which is transversely rubbed is negatively and the other positively electrified. If two bodies of the same substance, of the same polish, but of different temperatures, be rubbed together, that which is most heated is negatively electrified. Generally speaking, the particles which are most readily displaced are negatively electrified.In the following list, which is mainly due to Faraday, the substances are arranged in such order that each becomes7 positively electri-
fied when rubbed with any of the bodies following, but negatively when rubbed with any of those which precede it:
1. Catskin.
2. Flannel.
3. Ivory.
4. Rock crystal.
5. Glass.
6. Cotton.
9
7. Silk.
8. The hand.
9. Wood.
10. Metals.
11. Caoutchouc.
12. Sealing wax.
13. Resin.
14. Sulphur.
15. Gutta-percha.
16. Gun cotton.
The Charge.--The quantity of electrification of either kind produced by friction or other means upon the surface of a body is spoken of as a charge, and a body when electrified is said to be charged. It is clear that there may be charges of different values as well as of either kind. When the charge of electricity is removed from a charged body it is said to be discharged. Good conductors of electricity are instantaneously discharged if touched by the hand or by any conductor in contact with the ground, the charge thus finding a means of escaping to earth. A body that is not a good conductor may be readily discharged by passing it rapidly through the flame of a lamp or candle; for the flame instantly carries off the electricity and dissipates it in the air.
Distribution of the Charge.--When an insulated sphere of conducting material is charged with electricity, the latter passes to the surface of the sphere, and forms there an extremely thin layer. The distribution of the charge then, depends on the extent of the surface and not on the mass.
Boit proved that the charge resides on the surface by the following experiment:8
A copper ball was electrified and insulated. Two hollow hemispheres of copper of a larger size, provided with glass handles, were then placed near the sphere, as in fig. 4. So long as they did not touch the sphere, the charge remained on the latter, but if the hemispheres touched the inner sphere, the whole of the electricity passed to the exterior, and when the hemispheres were separated and removed the inner globe was found to be completely discharged.
The distribution of a charge over an insulated sphere of conducting material is uniform, provided the sphere is remote from all
other conductors and electrified bodies.
Fig. 4.--Boit's experiment which proved that the charge resides on the surface.
Figs. 5 to 8 show, by the dotted lines, the distribution of a charge for bodies of various shapes. Fig. 6 shows that for elongated bod-ies, the charge collects at the ends.
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.
