suspended in such a way that it is free to set itself parallel to the lines of force of the earth's field, then, owing to secular change in the declination and in the dip, the north pole of the magnet would describe a curve in space. The form of the curve in the case of a magnet in London is shown

in Fig. 429. From this curve, and similar ones drawn for other places, Bauer was able to show that the north end of such a freely suspended needle describes a curve such that, to an observer situated at the centre of the needle, the curve is described in the same direction as that in which the hands of a watch move. The form of the curve given in Fig. 429

seems also to indicate that the curve described by the pole of the needle will be closed, the time taken for the needle to complete a whole cycle being about 470 years.

437. Magnetic Storms.-In addition to the regular changes in the magnetic elements which we have been considering, sudden disturbances of these elements sometimes occur, which are often, especially when the

phenomenon called the aurora borealis is seen, of considerable magnitude. The character of such magnetic storms, as they are called, is shown by the copy of the photographic trace of the self-recording declination magnetograph of Greenwich Observatory during a magnetic storm and

also during an ordinary quiet day, reproduced in Fig. 430. The cause of these magnetic storms has not yet been discovered, although there seems to be some connection between them and the condition of the sun, for whenever there are a large number of spots on the sun there always seem to be a number of magnetic disturbances.

PART II-ELECTRO-STATICS

CHAPTER III

ELECTRO-STATIC ATTRACTION AND REPULSION—
COULOMB'S LAW

438. Fundamental Experiment.-Thales, who lived about the commencement of the Christian era, discovered that amber when rubbed acquires the property of attracting light bodies, such as pieces of pith or cork. Towards the end of the sixteenth century Gilbert showed that this property was also possessed by other bodies, such as wax, sulphur, and glass. All such phenomena are studied in the science of electricity, the name being derived from the Greek name for amber.

A body which has acquired this property of attracting other bodies, the attraction considered being of course different from the gravitational attraction which all bodies exert one on the other, is said to be electrified, or to possess electrification. Electrification, unlike mass, is not a fundamental property of matter, since under ordinary circumstances matter is unelectrified, and it is only after the electrification has been produced by certain causes, which we shall examine in detail later on, that it becomes electrified.

The most usual manner of causing the electrification of a body is that referred to above, namely, friction with a suitable rubber. Thus a stick of sealing-wax, when rubbed with a dry piece of flannel, becomes electrified, as also does a rod of glass when rubbed with silk.

439. Conductors and Non-Conductors.-All substances may be roughly divided into two classes, called conductors and non-conductors. In a conductor the electrification spreads all over the body, so that if one point of the body is by any means electrified, this electrification immediately spreads all over the body. In the case of a non-conductor, or insulator, as such bodies are also called, the electrification does not spread in this way, but remains in the neighbourhood of the point where the electrification took place.

The best conductors are the metals and solutions of most salts in water, while the best non-conductors are ebonite, glass, shellac, sulphur, paraffin, sealing-wax, and silk. There is, however, no hard and fast line of demarcation between the two classes, for such bodies as dry wood

and paper have intermediate properties, and are sometimes called semi-conductors. In the study of electricity it is of much importance to have a good non-conductor, for by this means we are able to support a body in such a way that any electrification communicated to it will not spread to neighbouring bodies through the support. Although no body is known which is a perfect insulator, yet glass, particularly when it has been boiled in water and is then kept in a dry atmosphere, paraffin, and fused quartz are sufficiently good insulators for all practical purposes. When a body is supported on an insulating stand, we shall speak of it as being insulated.

440. Two Kinds of Electrification. If a rod of sealing-wax is electrified by rubbing with flannel, and is then suspended by an insulating thread, such as silk, and a second rod of sealing-wax is also electrified in the same way and brought near the first, they will repel each other. We have here a case then of two electrified bodies repelling one another. In the same way, if two rods of glass are electrified by being rubbed with silk, and one of them is suspended by the silk thread and the other brought near, repulsion will take place. If, however, a rod of sealing-wax, electrified by friction with flannel, is brought near the glass rod, which has been electrified by friction with silk, the two will attract one another. We thus see that we have here to do with two kinds of electrification, in the same way that in the case of magnets we had to do with two kinds of poles. The kind of electrification that is developed in glass when it is rubbed with silk is distinguished by being called positive electrification, while the kind of electrification produced in sealing-wax by friction with flannel is called negative.

We may then state the law of electrical attraction and repulsion as follows: Bodies electrified in the same manner repel one another, while bodies electrified, one positively, and the other negatively, attract one another.

Whenever electrification of one kind is produced in any way, electrification of the opposite kind is also produced at the same time. Thus in the case of the glass electrified by friction with silk, while the glass will attract a negatively electrified rod of sealing-wax, the silk used to rub the glass will repel the sealing-wax, thus indicating that the silk has become negatively electrified.

The kind of electrification developed in a body depends on the nature of the body with which it is rubbed; thus while glass becomes positively electrified when it is rubbed with silk, it becomes negatively electrified when it is rubbed with a cat's skin. The kind of electrification produced is also dependent on the state of polish of the surface, on the temperature, &c.

441. The Gold-Leaf Electroscope.-In order to study the sign, and to a certain extent the magnitude of the electrification produced in a given body, the instrument shown in Fig. 431, and called the gold-leaf

D..

.E

A

electroscope, is often convenient. It consists of a glass flask, in which hang two gold leaves, CC, which are in conducting communication with a metal disc, A, by means of a metal rod, the rod being insulated by a coating of shellac, D. When the metal disc or cap of the instrument is put in conducting communication with an electrified body, the gold leaves both become electrified with the same kind of electrification as the body; and since two bodies electrified in the same way repel one another, they diverge as shown in the figure, the amount of the divergence being a rough measure of the amount of the electrification of the body.

B

FIG. 431.

442. Electrification by Induction.—If an electrified body is brought near the cap of a gold-leaf electroscope, it will be found that the leaves diverge, showing that they have become electrified before the electrified body has come into conducting communication with the cap. On the removal of the electrified body the leaves again collapse, showing that they have lost the electrification they possessed when the electrified body was near. This electrification, caused by the proximity of a charged body, is said to be produced by induction.

(From Watson's" Elementary Practical Physics.")

If the inducing body is charged positively, the part of the insulated body nearest to the inducing charge will be negatively electrified, while the part furthest from the inducing charge will be positively electrified. That this is so can easily be shown by means of a small piece of metal attached to an insulating handle, and called a proof-plane, which is brought into contact with different parts of the body on which the induced charges are produced. The sign of the charge carried away by the proof-plane, after contact with any given part of the body, can be found by means of the gold-leaf electroscope. In this way it can be shown that whenever an insulated conductor is placed in the neighbourhood of a charged body, the conductor will become electrified by induction, the electrification at the end nearest the charged body being of the opposite kind to that of the charged body, while the electrification on the end furthest from the charged body is of the same kind as that of the inducing charge. If, while an insulated conductor is in the neighbourhood of a charged body, so that it is charged by induction, it is placed in conducting communication with the earth, the electrification of the same kind as that of the inducing charge will be destroyed. If the connection with earth is now broken, and the inducing charge is then removed, it will be found that the conductor is now electrified with