electrical potential, we may add that in the system of electrostatic standards it has been found most convenient to adopt the centimetre as the standard of length, the gramme as that of mass, and the second as that of time: the system is therefore sometimes called the Centimetre-gramme-second System of Units.

We shall now give certain definitions founded upon this system :

(1). The unit of force is the force which will produce a velocity of one centimetre per second in a free mass of one gramme by acting on it for one second.

(2). The unit of work is the work performed by unit of force when it has moved a body over one centimetre.

(3). The unit quantity of electricity is that which exerts the unit of force on a quantity equal to itself at a distance of one centimetre across air.

(4). The unit difference of potential, or unit electro-motive force, exists between two points when the unit of work is spent by unit of electricity in moving from the one to the other against electric repulsion.

(5). The electric density signifies the quantity of electricity per square centimetre on a charged conductor.

(6). The capacity of a conductor is the quantity of electricity necessary to give it unit difference of potential.

(7). The co-efficient by which the capacity of an air condenser must be multiplied in order to give the same capacity when another dielectric is used is called the specific inductive capacity of the dielectric.

CHAPTER VIII.

ELECTRICITY IN MOTION.

LESSON XL.-MAGNETISM.

357. There is a certain kind of iron ore which has the property of attracting iron.

This circumstance was known to the ancients; and as the ore was first found in Magnesia, a place in Asia Minor, this gave rise to the term Magnet.

It will be afterwards shown how a long steel bar may be made into a magnet much more powerful than any which occurs in nature; the properties of a magnet may therefore be advantageously studied in such a bar.

B

FIG. 118.

358. Properties of a Magnet.—If A B denote a long steel magnet, and a small bit of soft iron be brought near it, the iron will be attracted to it, especially near the extremities of the bar; there will however be no attraction at the middle of the bar.

In like manner iron filings may be made to attach themselves to such a magnet, and they will cluster round the extremities as in the figure, but they will show no disposition

The appear

to attach themselves to the centre of the bar. ance of the iron filings leads us at once to the belief that there are two centres of force in the magnet, one near each extremity, and these two points are called the poles of the magnet. Again, if the magnet itself be suspended horizontally by a thread, it will be found to point with its length in a definite direction, to which it will always return, resisting every attempt to twist it away. This direction is, in Great Britain, very nearly north and south, and the pole which points to the north may, for convenience sake, be marked and termed the marked pole, to distinguish it from the other. It is sometimes called the north pole of the magnet, but this name is apt to create confusion.

359. Magnetic and Diamagnetic Bodies.-Let us now suppose that we have got a very powerful steel magnet; a small bit of soft iron will, as we have seen, be attracted to either of its poles. This attraction for iron will be very strong if the magnet be powerful, and it has been discovered by Faraday that many other substances will be attracted by such a magnet, only not so strongly as a bit of iron. Thus among the metals, nickel, cobalt, manganese, platinum, osmium, and palladium are magnetic, and so also are paper and sealing-wax among non-metallic substances.

are

On the other hand, Faraday found that certain substances are repelled by such a magnet; these he termed diamagnetic. Among the metals, bismuth, antimony, zinc, tin, mercury, lead, silver, copper, gold, and arsenic diamagnetic, being repelled from the poles of a strong magnet. Rock-crystal, phosphorus, and sulphur are also repelled. This repulsion is most marked in bismuth, but it is very much less strong than the attraction exerted between the magnet and iron.

In fact, most bodies are either sensibly attracted or repelled by the pole of a powerful magnet, but its attraction for iron is much stronger than its attraction or repulsion for any other body.

360. Suppose now that we place the marked pole of a powerful magnet near the unmarked pole of another. or.

what is a better arrangement, have one powerful magnet shaped so that its two poles come close together.

If we

Let A and B (Fig. 119) denote the two poles of such a magnet, and let there be an arrangement by means of which we can suspend different bodies midway between the two poles. first of all suspend a needle or slip of iron, it will point axially, that is to say, its length will lie in the line joining the two poles, as in the figure.

A

FIG. 119.

If, however, the needle be made of bismuth and not of iron, it will not point axially, but transversely or equatorially, that is to say, its length will be in a line at right angles to that

joining the two poles, and generally it will be found that all magnetic substances, or those which are attracted to a single pole, will point axially when placed between two poles, while all diamagnetic substances, or those repelled by a single pole, will point equatorially. In fact, a magnetic substance placed beween two such poles will endeavour to get as near to these poles as possible, so that it will place itself axially; while, on the other hand, a diamagnetic substance will keep as far away as possible, so that it will place itself equatorially or transversely.

361. It is instructive to observe what will take place if the substance is suspended in a magnetic liquid instead of in air.

Thus mica is magnetic, and in air will point axially; and a solution of protochloride of iron is also magnetic, and to a greater degree than mica. Now what will happen if the slip of mica be suspended in a solution of protochloride of iron and then exposed to magnetic influence? In this case, instead of pointing axially, it will point equatorially; that is to say, a magnetic substance suspended in a fluid more magnetic than itself will appear to be diamagnetic. And in like manner a diamagnetic substance suspended in a liquid more diamagnetic than itself will appear to be magnetic.

362. Action of Magnets upon each other.- Having thus described the action of magnets upon bodies in general, let us now proceed to their action upon one another.

Suppose that we swing a small magnet, suspended by a thread, and that we cause a powerful magnet to approach it. We shall find that the marked pole of the small magnet will be repelled by the similar pole of the large one, while it will be attracted by the opposite or unmarked pole. In fine, we have here a law similar to that which holds in electrified bodies, in consequence of which like poles repel, while unlike poles attract each other.

Coulomb has applied his torsion balance in order to discover the law of magnetic attraction and repulsion, and he finds that the forces exhibited vary inversely as the square of the distance.

Thus in the following figure, if we suspend a small magnet (a b), and cause to approach it the pole A of a large

A

FIG. 120

magnet (a and A being similar poles), we shall first of all have a repulsion exerted between A and a proportional

and we shall have an attraction between A and b

I

proportional to (Ab)2.

The consequence will be that the small magnet will point as in the figure; that is to say, the pole b will place itself opposite A, and not only so, but the small magnet, if it be free to move, will have a tendency to rush bodily to the large magnet, its tendency being represented by the excess of the