CHAPTER II

STATIC ELECTRICITY

Static electricity may be defined simply as _electricity at rest_; the term properly applies to an isolated charge of electricity produced by friction. The presence of static electricity manifests itself by _attraction_ or _repulsion_.

=Electrical Attraction and Repulsion.=--When a gla.s.s rod, or a stick of sealing wax or sh.e.l.lac is held in the hand and rubbed with a piece of flannel or cat skin, the parts will be found to have the property of attracting bodies, such as pieces of silk, wool, feathers, gold leaf, etc.; they are then said to be _electrified_. In order to ascertain whether bodies are electrified or not, instruments called _electroscopes_ are used.

There are two opposite kinds of electrification:

1. Positive; 2. Negative.

Franklin called the electricity excited upon gla.s.s by rubbing it with silk _positive_ electricity, and that produced on resinous bodies by friction with wool or fur, _negative_ electricity.

The electricity developed on a body by friction depends on the rubber as well as the body rubbed. Thus gla.s.s becomes negatively electrified when rubbed with catskin, but positively electrified when rubbed with silk.

[Ill.u.s.tration: FIGS. 2 and 3.--Pith ball pendulum or electroscope; the figures ill.u.s.trate also electrical attraction and repulsion.]

The nature of the electricity set free by friction depends on the degree of polish, the direction of the friction, and the temperature. If two gla.s.s discs of different degrees of polish be rubbed against each other, that which is most polished is positively, and that which is least polished is negatively 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 becomes positively electrified 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. Gla.s.s.

6. Cotton.

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 quant.i.ty 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 pa.s.sing 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 pa.s.ses 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 ma.s.s.

Boit proved that the charge resides on the surface by the following experiment:

A copper ball was electrified and insulated. Two hollow hemispheres of copper of a larger size, provided with gla.s.s 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 pa.s.sed 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.

[Ill.u.s.tration: 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 bodies, the charge collects at the ends.

The effects of points is ill.u.s.trated in fig. 9; when a charged body is provided with a point as here shown, the current acc.u.mulates at the point to such a high degree of density that it pa.s.ses off into the air, and if a lighted candle be held in front of the point, the flame will be visibly blown aside.

Fig. 10 shows an _electric windmill_ or experimental device for ill.u.s.trating 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_.

[Ill.u.s.tration: FIGS. 5 to 8.--Ill.u.s.trating 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.

=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.

Strictly speaking, there is no substance which will prevent the pa.s.sage of electricity, hence, the term non-conductors, though extensively used, is not correct.

[Ill.u.s.tration: FIG. 9.--Experiment to ill.u.s.trate the effect of pointed conductors.]

[Ill.u.s.tration: 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 bra.s.s 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.

=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_.

[Ill.u.s.tration: FIG. 11.--Gold leaf electroscope; it consists of two strips of gold foil suspended from a bra.s.s rod within a gla.s.s 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 gla.s.s jar, which both serves to protect them from draughts of air and to support them from contact with the ground. A piece of varnished gla.s.s tube is pushed through the cork, which should be varnished with sh.e.l.lac or with paraffin wax. Through this pa.s.ses a stiff bra.s.s 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 bra.s.s k.n.o.b.

When kept dry and free from dust it will indicate excessively small quant.i.ties of electricity. A rubbed gla.s.s rod, even while two or three feet from the instrument, will cause the leaves to repel one another. If the k.n.o.b be brushed with only a small 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:

[Ill.u.s.tration: 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 k.n.o.b with a gla.s.s 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 still 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 quant.i.ty of electricity thus imparted to them is greater.

[Ill.u.s.tration: FIGS. 13 and 14.--Electrification produced by rubbing dissimilar bodies together and then separating them. If the insulated gla.s.s 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 s.p.a.ce 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 s.p.a.ce, 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 foil and went inside with 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.