polar properties; those parts of the surface which are away from the north pole acquire north polar properties. An attraction is set up between the north pole of the magnet and the south polar side of the induced magnet, a repulsion of weaker amount between the north pole and the north polar side, so that on the whole the magnetic body is attracted to the north pole. This may even be the case sometimes when the magnetic body is itself a somewhat weak magnet, with its north pole turned to the given north pole. These two north poles would naturally repel each other; but, under the circumstances, the given pole will induce south polar properties in the north end of the weak magnet, and this south polarity may be greater than the original north polarity of the magnet, so that the two, the given north pole and the north end of the given magnet, may actually attract each other.

69. Experiments with Magnets.

(a) To magnetise a Steel Bar.

We shall suppose the magnet to be a piece of steel bar about 10 cm. in length and o'5 cm. in diameter, which has been tempered to a straw colour. The section of the bar. should be either circular or rectangular.

We proceed first to shew how to determine if the bar be already a magnet. We may employ either of two methods. Take another delicately-suspended magnet-a well-made compass needle will do-but if great delicacy be required, a very small light magnet suspended by a silk fibre. A small mirror is attached to the magnet, and a beam of light, which is allowed to fall on it, is reflected on to a screen; the motions of the magnet are indicated by those of the spot of light on the screen, as in the Thomson reflecting galvanometer. Bring the bar into the neighbourhood of the suspended magnet, placing it with its axis east and west and its length directed towards the centre of the magnet, at a distance of about 25 cm. away. Then, if N s be the suspended

magnet, N' s' the bar, and if N' be a north end, N s will be deflected as in fig. 50 (1).

(1)

F.G. 50.

S'

N

end, s' a south

On reversing

N's' so as to bring it into position (2), NS will be deflected in the opposite direction. If the action N/between the two be too small to produce a visible permanent deflexion of the magnet N S, yet, by continually reversing the bar at intervals equal to the time of oscillation of the needle, the effects may be magnified, and a swing of

considerable amplitude given to the latter. The swing can be gradually destroyed by presenting the reverse poles in a similar way.

This is a most delicate method of detecting the magnetism of a bar, and there are few pieces of steel which will not shew some traces of magnetic action when treated thus.

FIG. 1.

The following is the second method. Twist a piece of copper wire to form a stirrup (fig. 51) in which the magnet can be hung, and suspend it under a bell-jar by a silk fibre, which may either pass through a hole at the top of the jar and be secured above, or be fixed to the jar with wax or cement. If the magnet to be used be rectangular in section, the stirrup should be made so that one pair of faces may be horizontal, the other vertical when swinging. For very

delicate experiments this fibre must be freed from torsion. To do this take a bar of brass, or other non-magnetic material, of the same weight as the magnet, and hang it in

the stirrup. The fibre will untwist or twist, as the case may be, and the bar turn round, first in one direction then in the other. After a time it will come to rest. The fibre is then hanging without torsion. Now remove the torsion-bar and replace it by the magnetic bar which is to be experimented on, without introducing any twist into the fibre.

As the stirrup will be frequently used again for suspending the magnet, make a mark on the latter so that it can always be replaced in the same position on the stirrup.

If now the bar is at all magnetised, it will, when left to swing freely, take up a position of equilibrium with its north end pointing to the north, and when displaced from that position, will return to it again after a number of vibrations about it. This method would be even more delicate than the last, except that the torsion of the fibre might sometimes make it appear that the bar is magnetised when it is really

not so.

Having satisfied yourself that the bar is only feebly magnetised, proceed to magnetise it more strongly.

This can be done by stroking it with another magnet, using the method of divided touch, or by the use of an electric current. In

FIG. 52.

$4

$3

NA

S

N

$1

N2

Sa

the method of divided touch the bar is placed on two magnets NIS1, N2S2, Fig. 52; two other magnets are held as in the figure N3S3 N and N4S4. They are then drawn outwards from the centre slowly and regularly, from the position shewn in the figure, in which they are nearly in contact, to the ends. The operation is repeated several times, stroking always from the centre to the ends. Then the bar to be magnetised is turned over top to bottom and again stroked.

It will be found to be a magnet with its north pole N

over s1 and its south pole s over N ̧. In all cases the two ends of the bar rest on opposite poles, and the poles above, which are used for stroking, are of the same name as those below, on which the bar rests. The two magnets used for stroking should have about the same strength.

If an electric current be used, the bar may be magnetised either by drawing it backwards and forwards across the poles of an electro-magnet, or by placing it inside of a long coil of thick insulated wire, such as is used for the coils of an electro-magnet, and allowing a powerful current to pass through the wire.

It will be much more strongly magnetised if it be put into the coil when hot and allowed to cool rapidly with the current circulating round it.

To deprive a steel bar entirely of its magnetism is a difficult matter. The best plan is to heat it to a red heat and allow it to cool gradually, with its axis pointing east and west. If it be placed north and south, it will be found that the magnetic action of the earth is sufficient to re-magnetise the bar.

(b) To compare the Magnetic Moment of the same Magnet after different Methods of Treatment, or of two different Magnets.

(1) Suspend the magnet in its stirrup under the bell jar, as in fig. 51, and when it is in equilibrium make a mark on the glass opposite to one end. Displace the magnet slightly from this position, and count the number of times the end crosses the mark in a known interval of time,' say one minute-a longer interval will be better if the magnet continue swinging. Divide this number by the number of seconds in the interval, 60 in the case supposed, the result is the number of transits in one second. Call this n. There will be two transits to each complete oscillation, for the period of an oscillation is the interval between two consecutive passages of the needle through the resting point in the same direction, and all transits, both right to left The times of crossing the mark must be counted o, 1, 2, .. n.

and left to right, have been taken; n is therefore the number of complete oscillations in one second, and the periodic time is found by dividing one second by the number of oscillations in one second. Hence, T being the

Now K depends only on the form and mass of the magnet, which are not altered by magnetisation; H is the strength of the field in which it hangs, which is also constant; so that if M1, M2, &c. be the magnetic moments after different treatments, #1, #2, &c. the corresponding number of transits. per second,

We thus find the ratio of M, to M

(2) We can do this in another way as follows :-—

Take a compass needle, A B (fig. 53) provided with a divided circle, by means of which its direction can be determined, and note its position of equilibrium. Place the magnet at some distance from the compass needle, with its end pointing towards the centre of the needle and its centre east or west of that of the needle. Instead of a compass needle we may use a small magnet and mirror, with a beam of light reflected on to a scale, as already described (p. 453). The centre of the magnet should be from 40 to 50 cm. from the needle. The needle will be deflected from its position of equilibrium. Let the deflection observed be 1; reverse the magnet so that its north pole comes into the position