dered permanent. It is not absolutely necessary that the bars should be placed as directed, for in any direction not at right angles to the free dipping needle, magnetism will be imparted, though much slower, by induction from the earth. This is the reason why workmen's tools are so often found magnetic.

33. V. By Electrical Action.-Ampere found that by placing a bar of iron or steel in a helix or coil of insulated wire, and either discharging a Leyden jar through the coil, or by allowing a current of voltaic electricity to pass through it, the bar was rendered magnetic. The iron, as soon as it is withdrawn from the coil, loses its magnetic properties, but the steel from its coercive force retains them. An easier method is by coiling several thousand yards of insulated wire around a large bar of soft iron, made in the form of a horseshoe, and sending a galvanic current through it. This produces an exceedingly strong magnet, which may then be readily used as in the first method, and is found very effective.

34. Saturation Point.-It has been proved by experiment that when magnets are newly made, more power can be imparted to them than they can retain, and they gradually become weaker, until they reach a limit where the tendency of the fluids to reunite is exactly counterbalanced by the coercive force of the material used, and then the magnetism remains constant. This point is called the Saturation Point. If the magnetism imparted to magnets be not sufficient to reach that point, they will lose none after; hence more should be given to them than they can retain, that they may afterwards sink to their saturation point.

35. Armatures are pieces of soft iron connect

ing the poles of a magnet, by which its magnetic power is preserved. If a magnet be allowed to remain for some time without an armature, it loses its power, owing to the disturbing influence of the earth; but when it is provided with this appendage, the magnetism is kept in a state of constant activity, and the magnet effectually shielded from terrestrial disturbance. The action of armatures depends on magnetic induction, for the pole of a magnet induces a pole in the soft iron next it of a contrary name to itself; and thus, instead of being weakened by the earth's influence, the magnet is strengthened by having its poles in contact with poles of contrary names. Much greater weights can be sustained by magnets so armed than by the separate action of its poles. The armature adheres with greater tenacity to the magnet if it meets the latter with a rounded edge instead of with a plane surface. A compass needle, when free to move, does not require to be so guarded, because the earth itself acts the part of an armature.

36. Effects of Heat on Magnets. A magnet when heated loses power as it becomes hotter, and a part of it returns again as the magnet cools. This the Astronomer Royal proved by enclosing a magnet in a copper box, and surrounding it at different times with water of different temperatures. By this means he found what proportion of power was lost for a difference of one degree in temperature. At a bright red heat a magnet is completely demagnetised; but by ordinary temperatures, as that of the atmosphere, it is affected but little. The greater the power of a magnet, the less is it affected by heat. In soft iron, where the magnetism must be induced, the power increases with a rise of temperature to a blood red heat.

With other magnetic substances, as cobalt and nickel, the effects of heat vary and Faraday showed that some substances which at ordinary temperatures are not magnetic, become so when exposed to intense cold; and Grove has proved 'that whenever any metal susceptible of magnetism is magnetised or demagnetised, its temperature is raised.'

CHAPTER IV.

TO COMPARE THE STRENGTH OF MAGNETS-BY THE DEFLECTION OF A SMALL NEEDLE-BY THE

TORSION

BALANCE-BY THE METHOD OF OSCILLATIONS-ABSOLUTE UNIT OF FORCE.

37. (1) By the Deflection of a Small Needle. -The power of magnets is very conveniently compared by placing them end on to a small compass. The simplest method is to place the magnets to be compared successively at the same distance, magnetic east or west; then the powers are proportional to the tangents of the angles of deflection of the needle. Thus, if a magnet A, when so placed, causes a deflection of 20°, and a magnet B at the same distance similarly placed produces a deflection of 15° in the same needle, then

Power ofA: power of B:: tangent20° : tangent 15°. Or, if the magnets be successively placed at right angles to the needle when at its maximum deflection, the powers are proportional to the sines of the angles of deflection. Thus, if the greatest deflections be 25° and 18°, we get

Power of A: power of B : : sine 25° : sine 18°. This is a very convenient method of comparing the strength of compass needles.

Sir W. Snow Harris says that Lambert discovered the law, that 'the effect of each particle of the magnet on each particle of the needle, and reciprocally, is as the squares of the distances inversely; thus showing that magnetism follows the laws of all central forces. Coulomb, about twenty years after, proved the truth of these laws by means of the torsion balance and the method of oscillations. The process is as follows:—

Fig. 9.

38. (2) The Torsion Balance. This apparatus depends on the principle, that when a wire is twisted the force used is proportional to the angle through which the twist takes place. It consists of a glass cylinder, c, with a glass cover, in which is a hole, h, for the introduction of the magnet. In the centre of the top a glass tube, T, fits, having at its upper extremity a disc, D, graduated at its edge to 360°, and a pointer, movable by a button in its centre, to which is attached the very fine silver wire carrying the magnetic needle, n, s. On a level with n s is a scale graduated also to 360° to show the torsion of the wire. When about to be used, the pointer, D,

must be placed at zero, and the whole apparatus turned so that the needle, n s, shall also point to zero on its scale. Then to secure there shall be no torsion in the wire by the directive power of the needle, the cover is lifted and the needle, ns, removed, and its place supplied by a similar needle of copper or other non-magnetic substance of

equal weight. The cover is replaced exactly as before by pins, which fit into small notches on the edge of c. Then if the bar of copper does not point exactly to zero on the scale of c, the glass tube, T, is screwed round to ensure that the copper needle shall be in the magnetic meridian, and therefore pointing to the zero of the scale. On replacing n s it must now stand directly magnetic, north and south, and there can be no torsion in the wire: the apparatus is ready for use only when the pointer on D points to zero, and the needle, n s, points also to zero on its scale when there is no torsion in the silver wire by which the needle is suspended.

Before we can test the accuracy of the laws we must have some means of estimating the effects of the earth. This is done by seeing how many degrees the pointer on D must be moved to cause the needle, ns, to move through 1°. Coulomb found that in his torsion balance it required the pointer on D to move through 35° to effect a movement of 1o in n s. As the angle of torsion is proportional to the force used it required twice 35° movement of the pointer to cause ns to move over 2o, thrice 35° to move ns over 3°, and so on. Having determined the action of the earth and restored the apparatus to its proper position, a magnet, M, is inserted in the hole, h, and the deflexion of n s is noted, say 20°.

Then the force acting to bring the needle back into the magnetic meridian (if the instrument used be similar to Coulomb's) may be represented by 20° + (20 × 35)° = 720.

Next turn the button and pointer on D, so that the needle shall be deflected only 10°, and it will be found that it requires about seven complete re volutions in a contrary direction.