space around each of the poles is permeated by this influence, and is, therefore, known as the magnetic field. The strength

of the attraction within this field is governed by a law of distance similar to those already noted under heat and light. It is expressed as follows: Magnetic attraction or repulsion varies inversely as the square of the distance through which it acts.

IV. THE EARTH AS A MAGNET.

1. The Influence of the Earth upon Magnets. We have already seen that a magnetic needle if freely suspended will always, after oscillation, come to rest in a position pointing north and south. We will also find, if the needle be suspended so that it can swing in a vertical plane, that as we move from the equator toward the pole the needle will dip increasingly, and near the pole it assumes a vertical position. These two results we attribute to the earth's influence. We have effects analogous to these if a powerful bar magnet be laid upon a table and we hold suspended over it by a fine thread a delicate magnetic needle. The latter, as it approaches the large magnet, immediately assumes a position parallel to it, and as we proceed from the neutral line of the large magnet toward the pole the small needle dips toward the pole approached, and when immediately over it assumes a vertical position. If over the south pole of the large magnet, it is the pointed or north end of the needle which dips down, and, if over the north pole of the magnet, it is the unmarked or south end of the needle.

2. The Earth Itself a Magnet.-We are led by the analogy just stated to consider that the earth itself acts as a great magnet,

and that it is to its directive influence that the oscillating needle always tends to assume the north-and-south direction. If the earth be considered to act as a magnet, we should be able to locate its poles. These magnetic poles of the earth were, in fact, discovered by the navigator, Sir James Ross. The north magnetic pole was found on the western coast of Boothia in 70° north latitude, and a point in the South Sea was found where the inclination or dip of the needle was 88° 37', whence the position of the south magnetic pole was calculated to be in about 752° south latitude. It will be seen that the magnetic poles of the earth do not agree in position with the geographical poles. Hence the needle does not in most places point due north and south or in a line determined by the north star. The angle which the needle makes with the geographical meridian is known as the angle of declination. It is found that this angle differs at different places, and that for any one place it changes slightly from year to year. The magnetic axis of the earth, in fact, is slowly oscillating from east

to west.

3. Cause of Terrestrial Magnetism. The cause of the earth's magnetism is unknown. It is generally believed that it is due to electrical currents flowing from east to west, so that the earth is in effect an electro-magnet (see p. 97). These currents are undoubtedly influenced by the sun, as is shown by the coincidence of changes in the appearance of the sun and the outbreak of magnetic storms upon the earth. These latter at times disturb magnetic needles and telegraph wires over the whole world. The auroral displays seem also to be connected with these magnetic disturbances.

V. APPLICATIONS OF MAGNETISM.

Permanent magnetism has few applications compared with those that have been found for electro-magnets. These will be noticed farther on. The most important application of permanent magnets is in the mariner's compass. In this the freely oscillating magnetic needle is mounted in such way that under the influence of the earth's directive action it is made to indicate the direction in which a ship is moving. For this purpose one magnetic needles are attached to the under surface of a circular disk of card-board or mica, upon the upper surface of which are marked the thirty-two points of the compass. This disk is supported upon a delicate pivot moving in an agate cup, so that it responds to the slightest change of course of the ship.

or more

With the general introduction of iron and steel ships the use of the compass would seem to be attended with great difficulty, because of the local attraction for the needle. It has been found possible, however, to correct for this local influence so as not to interfere with the accuracy of the directive action of the needle. The compass is supposed to have been known to the Chinese long before its introduction into Europe in the twelfth century.

Permanent magnets, as well as electro-magnets, are also used in separating magnetic iron ore from accompanying sand and crushed rock material, and in picking out small particles of scrap iron from malt and grain before crushing.

CHAPTER VI.

ELECTRICAL ENERGY.

UNDER the influence of heat, magnetism, mechanical and chemical action, as well as by the action of natural forces, a form of energy may be developed in bodies to which the name of electricity has been given. This name was first given, some six centuries before Christ, to characterize the phenomena produced by rubbing bodies like amber (Greek, nextpov), and the identity of these with the atmospheric phenomena of lightning and thunder was only established toward the close of the last century by Franklin. We shall discuss electrical energy under two main heads-the electricity of friction and the electricity of chemical action-and then note the most important of its recent appli

cations.

A. ELECTRICITY OF FRICTION.

1. Conditions of Production.-If a stick of sealing-wax, sulphur, or hard resin be rubbed briskly with a piece of flannel, it will acquire the property of attracting light bodies, like slips of paper, feathers, pith balls, etc. A glass tube carefully dried and rubbed with a silk handkerchief will show similar phenomena. In a dark room the bodies so rubbed often appear slightly luminous, and sparks are sometimes given off when they are touched. It is evident that these are manifestations of a new form of energy developed by the friction of the two bodies and distinct from heat, which may be developed at the same time. The generally received explanation of this change of energy into a new form is that friction and the other means referred to disturb the neutral condition in which the molecules of bodies ordinarily exist, and develop therefrom two opposing influences, which then give us the so-called "electrical" phenomena. These influences may be designated as positive and negative, or vitreous and resinous electricity.

The development of electricity of either kind in a body is said to give it a certain potential as compared with another body, either electrified or unelectrified. A body charged with positive electricity is said to have a positive potential, while a body negatively

charged has a negative potential. These opposing charges may be equal, in which case, when the two bodies are brought into contact, they neutralize each other and leave the bodies unelectrified, or they may differ in potential, in which case a residue remains of the stronger charge.

If instead of rubbing resins or glass we rub metals with flannel or silk, apparently no electrical effects are produced. This want of electrical excitement is, however, only apparent. If a metal tube is provided with a glass handle by which it may be held and is then rubbed with silk or flannel, it becomes charged with electricity. Its seeming inability to become electrified is due to the fact that it is a conductor of electricity, and hence the charge passes off rapidly into the hand or the object with which it was in contact. We may, therefore, distinguish two classes: conductors, like metals, water and aqueous solutions, plants and animals, and non-conductors, like silk, glass, resins, rubber, and dry air. Non-conductors are also often called insulators, as they prevent the loss or dissipation of a charge of electricity accumulated upon a conductor. Glass and hard rubber are thus used in practice. We can now somewhat understand the result attained when a stick of shellac is rubbed with a piece of flannel. Both become electrified, the shellac taking the negative charge of electricity and the flannel the positive charge. On the other hand, when glass is rubbed with silk, both likewise are electrified, but the glass takes the positive electricity and the silk the negative electricity. Both these results are, however, explained if we arrange the substances referred to consecutively in what is known as a potential series. In this list the substances are named in such order that any one becomes positively electrified when rubbed with one of the bodies following, but negatively when rubbed with one of those which precede it :

2. Laws of Electrical Attraction and Repulsion.-While we have referred to the distinction between positive and negative electricity we have not shown how this different character in electrified bodies may be ascertained. If a stick of sealing-wax be rubbed with flannel or cat's skin, and after suspending it with