304.] MOLECULAR MOTION IN ELECTRO-MAGNETS. FIG. 241. 615 the helices. In preparing an electro-magnetic coil it is not necessary, however, that the wire be coiled in one direction only, if the wire be continuous; for instance, if the coils follow the direction of the thread of a right-handed screw in the first layer, as in a B, fig. 240, the wire in winding it backwards from B to A will be formed into a lefthanded helix, but this is of no consequence, because the direction of 0000 the current is also reversed in this layer, being now from в to A, so that the effect of the reverse twist of the wire is neutralized: in fact, if the currents in the turns of wire are parallel, the kind of helix is a matter of indifference. A helix through which an electric current is passing is powerfully magnetic: the two magnetisms accumulating at its opposite extremities. If the helix be supported with its axis in a vertical position, and a bar of soft iron be partially introduced within it, as soon as an electric current of sufficient strength is passed through the coils, the bar will start up, and will raise itself in mid-air nearly equidistant between the two extremities of the coil; the iron, by induction, becoming for the time a powerful magnet. The poles of the iron bar are the same as those of the helix by which its magnetism is produced. The most powerful electro-magnets, however, are those in which the iron is bent into the form of a horse-shoe, and around which an insulating conducting wire is coiled in several layers, with due attention to the direction in which the coils are arranged. In this way magnets have been constructed which are able to sustain a weight exceeding that of a ton (1016 kilogr.). The magnetism developed in the soft iron, under the influence of the voltaic current, attains its maximum in a few moments. It ceases as quickly, when the contact of the wires with the battery is broken; and, by reversing the direction of the current, the magnetic polarity of the bar is instantly reversed. (304) Molecular Movements during the Magnetization of Bars. -The production of magnetism in a bar of iron, and the cessation of magnetism, are both attended with molecular motion, 616 LAWS OF ELECTRO-MAGNETISM. [304. Joule (Phil. Mag. 1847, which pervades the whole mass of iron. XXX. 76 and 225) has shown that the bar, on becoming magnetic, acquires a slight increase in length, and suddenly contracts to its former dimensions when the magnetism ceases, the elongation of the bar being proportional to the square of the intensity of the magnetism developed within it. It has been observed by Guillemin that if an iron bar be supported at one end so as to bend by its own weight, it becomes straightened to a greater or less extent when magnetised. Wertheim has also observed that the co-efficient of the elasticity of both iron and steel is diminished by magnetization. Each time that the bar either becomes magnetic or loses its magnetism, a distinct sound is emitted, the note being similar to that elicited by striking one end of the bar so as to produce vibrations in a longitudinal direction. The molecular movements, if repeated in quick succession by rapidly making and breaking contact between the ends of the helix and the wires of the battery, so as repeatedly and quickly to magnetize and demagnetize the bar, produce an elevation of temperature, which, as Grove has shown, is quite independent of the heat produced in the conducting wire by the current. In connexion with these molecular movements, it may be noted that Wiedemann finds when a current is transmitted along the axis of a magnet, the magnet suffers a slight degree of twisting. Gore (Phil. Trans. 1874, 529) has investigated the effect of passing a current through a piece of iron wire about 2 metres long, hanging vertically, and magnetized by a helix of copper wire surrounding it. When the current passes along the wire from the south-seeking to the north-seeking end, a right-handed torsion results; and when either the current or the magnetization is reversed, the torsion is ⚫ left-handed. A similar effect was observed with a bar of nickel, but no motion was observed with platinum, silver, copper, lead, tin, cadmium, zinc, magnesium, aluminium, brass, or German silver. A stretched cord of gutta-percha showed no signs of torsion when placed within the helix. (305) Laws of Electro-Magnetism.-According to the researches of Lenz and Jacobi (Pogg. Ann. 1839, xlvii. 225, and 1844, lxi. 254) it appears that if the battery current be maintained of a uniform strength-1. That the magnetism which is induced in any given bar of soft iron is directly proportioned to the number of coils which act upon the bar: it is a matter of indifference whether the coils be uniformly distributed over the whole length of the bar, or whether they be accumulated towards its two extremities. 2. That the diameter of the coils which surround 305.] LAWS OF ELECTRO-MAGNETISM. 617 the bar of small diameter scarcely influences the result, provided that the current be in all cases of uniform strength; for although the inductive influence decreases as the distance of the magnet from the wire, the induction produced by the increased length of the wire in the circumference of the coil is augmented in precisely the same ratio. 3. That the thickness of the wire composing the coil does not influence its effect upon the bar. 4. That the intensity of the magnetism is, cæteris paribus, proportioned to the strength of the current, being directly as the electro-motive force and inversely as the resistance of the circuit.* 5. That the retentive power of the magnet, like the attractive power in electricity, increases as the square of the intensity of the magnetism. 6. That the intensity of the magnetism induced upon a solid bar by a given current is proportioned to the surface which the bar exposes; or in cylindrical bars it is as the square root of the weight. Bundles of isolated wires expose a larger surface than a solid bar, and hence they are susceptible of a higher amount of magnetism than a solid bar of equal weight. 7. That the employment of long bars has no other advantage over the use of short bars than that of removing to a greater distance the counteracting influence of the two magnetic poles upon each other. According to Du Moncel the larger the armature, until it equals the electro-magnet in size, the more powerfully is it attracted. The practical question in preparing an electro-magnet resolves. itself into the determination of the thickness and length of the wires which are required to produce the maximum effect. It is obvious, that for a battery of a given power, the longer the wire which is employed, the greater is the resistance introduced, so that practically the number of convolutions has a limit beyond which nothing is gained by increasing them, and this limit is attained when the increased resistance introduced by the increasing length of the wire balances the gain produced by the influence of *This increase, it must be observed, only occurs up to a certain point, as there is a limit to the amount of magnetism which can be developed in iron, although the electricity may be indefinitely increased. + Dub (Pogg. Ann. 1858, civ. 234), however, confirms the observation of Müller, which gives a different result-viz., that the intensity of the magnetism in cylindrical bars is, for equal currents in coils of equal number, proportioned to the square root of the diameter of the bar; the magnetism developed in a bar 10 centimetres thick being twice as intense as that produced in a bar of 2.cm.5 in thickness; so that the retentive power is directly proportioned to the diameter of the bars. 618 AMPÈRE'S THEORY OF ELECTRO-MAGNETISM. [305. the additional coils upon the bar; the greater the diameter of the coil the longer, of course, will be the wire required to form it, and the greater will be the resistance of such a coil in proportion to its magnetizing power. Experience shows, that in order to attain the most economical combination in the battery in proportion to the quantity of materials consumed, when magnetic power is required, the same rule must be followed as when chemical resistance has to be overcome-viz., that that combination is the most effective in which the resistance of the wires and of the coils which are exterior to the battery is equal to the resistance of the liquids and other materials used in the construction of the battery itself, or when in Ohm's Formula (4) the value of A most nearly approaches 0'5; in which case r=nR. + r (306) Ampère's Theory of Electro-Magnetism.-It will be necessary to examine somewhat further the properties of a helical wire which is conveying a current, in order that the reader may be enabled to understand the theory of Ampère, by which he accounted for the mutual action of magnets and electric currents. If a simple helix, which for lightness may be made of thin wire, be freely suspended, it will, whilst conveying the current, place itself in the magnetic meridian; that is to say, it will point north and south, and will be attracted and repelled by a magnet which is presented to it, just as an ordinary bar-magnet would be. Fig. 242 shows a method of suspending the helix, or electro-dynamic cylinder, n s, so as to exhibit these effects; the wire, a, terminates in a small hook, which dips into a cup containing mercury, and this is connected with one of the wires from a small voltaic battery; the other end, b, of the coil dips into a second mercury cup, which is in communication with the other wire of the battery: the magnetism corresponding with that of the north end of the needle accumulates at one extremity of the coil, whilst the opposite magnetism accumulates at the other extremity: this effect necessarily follows from the influence of each coil upon its neighbours, since the north side of every coil is in one direction, whilst the south side is in the opposite. Ampère, who first pointed out the remarkable analogy between an ordinary magnet 307.] MUTUAL ACTION OF VOLTAIC CURRENTS. 619 and the helix when conveying an electric current, deduced from it a theory of the connexion between magnetism and electricity which has satisfied, hitherto, the rigorous requirements of mathematical analysis, and has also explained all the phenomena of electro-magnetism that have as yet been discovered. Ampère assumes that all bodies which exhibit magnetic polarity, derive this polarity from currents of electricity which are perpetually circulating around the particles of which the magnetic bodies are composed. Around each particle an electric current is supposed continually to circulate; the direction of these currents is conceived to be uniform, each current circulating in a plane at right angles to the magnetic axis of the particle. In fig. 243, the currents are shown as at a, b, c, circulating in a uniform direction around the particles of a bar magnet, of FIG. 243. which the south pole, s, is nearest the observer. The resultant effect of these united and concordant small currents would be equivalent to that produced by a single current winding in a helical direction uniformly around the bar which would occupy the axis of such a helix. In an ordinary magnetic needle which is pointing north and south, currents would ascend on the western side and descend on the eastern. So that if the south pole is towards the observer, the direction of the current required to produce the magnetism will be the same as that of the hands of a watch with its face upwards. No definite proof of the existence of these currents can be given, nor can a reason for the persistence of such currents in permanent magnets be assigned; but granting that such currents do exist, all the mutual actions, between wires which convey currents, and permanent magnets, follow as a matter of necessity. (307) Mutual Influence of Wires which are conveying Currents. -We proceed to point out one or two of these consequences. When two wires are freely suspended near each other, and electrical currents are passed through them, the wires will be mutually repulsive if the currents pass in opposite directions, but they will attract each other if the currents be in the same direction. Fig. 244 will explain the reason. When the currents are in opposite directions (No. 1), the magnetism on the inner side of the first wire is exactly similar to that on the contiguous side of the second wire, as indicated by the arrows arranged round P and N. The two north poles and the two south poles consequently |