Current Electricity.--This may be defined as the quantity of electricity which passes through a conductor in a given time--or, electricity in the act of being discharged, or electricity in motion.
An electric current manifests itself by heating the wire or conductor; by causing a magnetic field around the conductor and by causing chemical changes in a liquid through which it may pass.
Dynamic Electricity.--This term is used to define current electricity to distinguish it from static electricity.
Radiated Electricity.--Electricity in vibration. Where the current oscillates or vibrates back and forth with extreme rapidity, it takes the form of waves which are similar to waves of light.
Positive electricity.--This term expresses the condition of the point of an electrified body having the higher energy from which it flows to a lower level. The sign which denotes this phase of electric excitement is +; all electricity is either positive or negative.
Negative Electricity.--This is the reverse condition to the above and is expressed by the sign or symbol -. These two terms are used in the same sense as hot and cold.4
Atmospheric Electricity is the free electricity of the air which is almost always present in the atmosphere. Its exact cause is unknown. The phenomena of atmospheric electricity are of two kinds; there are the well known manifestations of thunderstorms; and there
are the phenomena of continual slight electrification in the air, best observed when the weather is fine; the Aurora constitutes a third
branch of the subject.
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Fig. 1.--The electric eel. There are several species inhabiting the water, and which have the power of producing electric discharges by certain portions of their organism. The best known of these are the Torpedo, the Gymnotus, and the Silurus, found in the Nile and the Tiger. The Electric Ray, of which there are three species inhabiting the Mediterranean and Atlantic is provided with an electric organ on the back of its head, as shown in the illustration. This organ consists of laminae composed of polygonal cells to the number of 800 or 1000, or more, supplied with four large bundles of nerve fibres; the under surface of the fish is -, the upper +. In the Surinam eel, the electric organ goes the whole length of the body along both sides. It is able to give a very severe shock, and is a formidable antagonist when it has attained its full length of 5 or 6 feet.
Frictional Electricity is that produced by the friction of one substance against another.
Resinous Electricity.--The kind of electricity produced upon a resinous substances such as sealing wax, resin, shellac, rubber or amber when rubbed with wool or fur. Resinous electricity is negative electricity.
Vitreous Electricity.--A term applied to the positive electricity developed in a glass rod by rubbing it with silk. This electric charge will attract to itself bits of pith or paper which have been repelled from a rod of sealing wax or other resinous substance which had been rubbed with wool or fur.
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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 glass rod, or a stick of sealing wax or shellac 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, feath-ers, 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 glass 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 glass becomes6 negatively
electrified when rubbed with catskin, but positively electrified when rubbed with silk.
Figs. 2 and 3.--Pith ball pendulum or electroscope; the figures illustrate 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 tempera-
ture. If two glass 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 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.
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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