it is desired to make a series of measurements at points along a particular line on the magnet.

When we wish to measure the force along the edge of the magnet, or along a line on the face of a very thick magnet, it is desirable to begin by loosening the screws E, E, which clamp the bar A to the vertical uprights c, then raising the bar A to clamp it higher up. In fact, the motion of the bar A up or down may be regarded as a coarse adjus'ment, and the turning of the nut N as a fine adjustment. As parallax will be diminished by keeping the distance between the dial and the pointer small, it is desirable to raise the bar A, when the bar A requires to be moved up. To make one adjustment sufficient it is convenient to connect the bars ▲ and A by tubes sliding on the uprights c c, as seen in the figure, so that the whole of the measuring apparatus can be moved up and down together when the screws E, E are loosened.

Before making a test the face of the magnet and the lower surface of the iron ball should be rubbed with fine emery-cloth and then wiped free from dust, and it is to be carefully remembered that all magnetic experiments on the force of detachment are much affected by the character of the surfaces of contact. It is therefore desirable to make each experiment several times.

If experiments be made at points equidistant from one another all along, say, the central line of the magnet, it will be found that the force exerted by the magnet on the ball is very large towards each end, rapidly diminishes as we approach the centre, and becomes practically nought at the middle of the magnet. If similar experiments be conducted along a line parallel to the long edge of the magnet, but much nearer to one edge than the other, similar results will be obtained, but the forces at the ends of the magnet will be even greater than before. If the magnet be "uniformly magnetised," the attraction of the iron ball will not indicate any difference between the forces at two points similarly situated relatively to

the two ends of the magnet; but if we approach our bar magnet, м M, to a suspended compass needle, we find that the north-seeking end of the compass needle is attracted by one end of the bar magnet and repelled by the other, and so for the south-seeking end of the compass needle.

Hence, although the forces exerted on a piece of soft iron at points symmetrically situated relatively to the two ends of a uniformly magnetised steel bar are the same in every respect, the forces exerted by the two ends of the large magnet on one end of a compass needle are opposite in character.

17. Magnetic Poles.-The experiment just described shows that the magnetic force exerted by a magnet on a piece of soft iron is greatest near the ends of the magnet, although every part of the magnet exerts some force. Now it is found that the longer a magnet is compared with its breadth, the more concentrated is the magnetism towards its ends; and when the magnet is, like a needle, very long and thin, all the force which it is capable of exerting is due to the action of the magnetism present at the two extreme points of the needle. These points at which the magnetism is concentrated are called the "magnetic poles" of the magnet, and the line joining the poles is called the "magnetic axis" of the magnet.

18. Why Magnetic Needles tend to Point North and South. If a piece of magnetised steel, ns (Fig. 33), be balanced in a paper stirrup, P, and be suspended by a fibre of unspun silk so that the steel wire or bar is free to turn horizontally, it will be found that its axis points towards a magnetic pole held near it. This is because the poles of the suspended bar are acted on by forces due to the neighbouring pole. This action is an attraction in the case of one of the ends of the suspended bar, and a repulsion in the other. In the same way, due to the magnetic field" exerted by the earth, the poles of any magnet are acted upon by a magnetic force which is directed northwards for one and southwards for the other pole. This force always exists, whether the magnet be

fixed or movable, and whether it is pointing northwards or in any other direction; but if the magnet be free to move easily, so that it can be turned by the application of a comparatively small force, and if it be acted on by the earth's magnetism alone, the axis of the suspended magnet is found to place itself north and south, and always with one particular end of the magnet towards

2

Fig. 33.

the north, which is therefore called the "north-seeking" end or pole of the magnet.

If this test be made with various magnets, first with one and then with the other, care being taken that the suspended magnet be not near any of the others, the north-seeking end of each magnet can be successively found and marked for future reference. Now, if any one of the magnets be suspended, and the ends of one of the other magnets be brought respectively near the ends of the suspended magnet, it will be found that marked poles

or unmarked poles repel one another, while a marked and an unmarked pole attract one another. Hence we are led to the general rule that similar poles repel, dissimilar attract.

19. Why a Galvanometer Needle has a Given Deflection for a Given Current.-A body moves, or changes the character of its motion, only when a mechanical force is exerted upon it. A magnetic needle

which is free to turn is found to deflect whenever a wire through which a current of electricity is flowing is brought near it, unless the wire be placed exactly at right angles to the suspended needle and the current be flowing in one particular direction along the wire. Hence this deflection, which always occurs except in the case just mentioned, must take place, because in some way or other a conductor carrying an electric current exerts a force on the poles of a magnetic needle.

If there be no other magnetic force acting on the needle than that due to the current, the needle, if free to turn, will place itself at right angles to the wire conveying the current, the north-seeking or marked pole (as was seen in § 7, page 34, Chapter I.) coming towards the observer if the current flows in such a direction as to form part of a counterclockwise circuit round the needle.

If, in addition to the forces acting on a needle due to the current, there be other forces due to one or more magnets, weights or springs, &c., then the needle, if free to turn in any direction, will place itself along the resultant of all the forces. And consequently the law of a particular galvanometer-that is, the law connecting the strength of the current with the magnitude of the deflection-depends on the way in which the direction of the resultant force varies with the current flowing through the galvanometer coil.

If, instead of allowing the magnetic needle to deflect more and more as the current is increased, it is always held in the same position relatively to the coil by the application of a force, the law of the instrument becomes

very simple, for as long as the current does not become so strong as to alter the magnetism of the needle the strength of the current is directly proportional to the force that has to be exerted to keep the needle at rest relatively to the coil; and this simple law is true whatever be the size and shape of the coil, and whatever be its position relatively to the magnetic needle.

But with small currents the force exerted on an ordinary magnet is too feeble to enable the current to be easily measured by measuring a force, and consequently with galvanometers the needle is generally allowed to deflect, and the magnitude of the current is ascertained from the amount of the needle's deflection.

If the magnetic field controlling the motion of the needle and tending to bring it back into the zero position be that produced by the earth, the force acting on the end of the magnet will be the same both in magnitude and direction for all positions of the magnet within a limited space, say that of an ordinary room. Such a field is called a "uniform magnetic field," and it can be produced not only by the earth, but by a magnet put far away. The magnetic field, for example, produced inside, say, the space of a cubic inch by a magnet put a few feet away, will be practically uniform. A very weak magnet put at a distance of an inch may produce as powerful, or even a more powerful, effect than a strong magnet at a distance of several feet. The former, however, will not produce a uniform field throughout a space of even a cubic inch. The condition, then, for the magnetic field throughout a space being a uniform one is that the linear dimensions of the space are small compared with the distance of the magnet from the nearest point of the space in question.

And in exactly the same way, as long as we are dealing with a space of, say, a few thousand cubic feet, the gravitational force with which a body is pulled to the earth is constant both in magnitude and direction. But when the body is removed from England to America this is no longer the case.