ELECTRIC), the field of an elec tromagnet is used for exerting force upon a wire in which a current of electricity is flowing.
The law of the tractive force of electromagnets, first announced correctly by Clerk Maxwell, states that the normal traction between two plane pole-faces, placed in contact and magnetized uniformly in a direction at right angles to their common interface, is equal to 7r where B is the flux density and A is the area of the surface of contact. The expression represents the total normal traction exerted across the area A, and in some cases this may be considerably greater than the force required to pull the surfaces apart. For example, if two iron bars of cross section A are passed through the borings of the electromagnet depicted in fig. 5, and placed so that the plane of contact of their ends is midway between the pole-pieces, the force required to separate the bars is approximately 7, the difference representing a portion of the attraction between the pole-pieces, which is of course not overcome when the bars are pulled apart. In an experiment conducted on these lines with the du Bois ring electromagnet, the area of the cross section of the bar was 0.1896 sq.cm., the value of B was 39,26o, and that of H was 17,690 c.g.s units. The value of 7r was 11.86, that of was 2.386, both expressed in kilogrammes weight. Thus it was 9.474, which agrees well with the observed pull which was 9•43kg. weight. In another experiment the pull between the bars was 1,6341b. weight per square inch of their cross section.
In ordinary cases, such as that of a horseshoe or mushroom shaped electromagnet supporting an iron mass attached to its poles, the lifting force is represented by per unit area of contact surface, and both poles are of course to be taken into account. From this expression we can find the effect of a change of dimensions of a lifting magnet, with proportional change of current and consequently no change of flux density, on the lifting power. Since the weight which it can support is proportional to the area of the contact surfaces, that is, to the square of the linear dimensions, while the weight of the magnet itself is proportional to their cube, it follows that the ratio of the lifting power of a magnet to the weight of the magnet itself is inversely propor tional to the linear dimensions. Thus, in proportion to their weight, small magnets can lift more than large ones. One of Joule's electromagnets weighed only half a grain and it could support 1,417 grains, or 2,834 times its own weight.
Turning now to the question of the force acting on a small magnetized body placed in a magnetic field, we find in the first place that if the field is uniform there is no resultant force on the body since its two poles are subjected to equal and opposite forces. The resultant force on the body must therefore depend upon the non-uniformity of the field. It acts in the direction in which the spatial variation of the field is greatest, urging mag netic bodies towards the stronger, and diamagnetic bodies towards the weaker parts of the field. (See MAGNETISM.) The force here considered has been put to practical use in the surgical electromagnets used for extracting small particles of iron or steel from the eye or other parts of the body. The same forces are employed in magnetic separators used for separating iron from other materials. In one machine the electromagnet is formed by the separator pulley, which contains a number of coaxial coils, the windings of which are near the curved surface of the pulley. The coils are separated from one another by steel discs, the rims of which, lying in the surface of the pulley, form the poles. The diagram in fig. 6 shows the mode of action of the separator pulley. The particles of magnetic material ad here to the belt where it is in contact with the pulley, but leave it after contact with the pulley is broken.
The force due to non-uniformity of the field is also used in cer tain scientific experiments, for example, in the determination of the magnetic susceptibility of weakly magnetic substances by Curie's method, and in the experiments of Gerlach and Stern for measuring the magnetic moments of atoms. (See MAGNETISM.) Electromagnetic Mechanisms.—There have been invented a very large number of devices in which there is a small movement of a piece of iron forming part of the magnetic circuit of an electromagnet due to causing the current to flow in the exciting coils. The movable iron usually forms the armature of the electro magnet, held by a spring near the poles but not in contact with them. In purely electromagnetic systems the armature is at tracted when the current flows in either direction. In many of these devices the electric circuit includes a contact breaker so arranged that the movement of the armature towards the electro magnet causes the circuit to be interrupted ; the core then be comes demagnetized and the armature is drawn back by the spring. In this way vibratory movement of the armature can be maintained by the current which is rendered intermittent by the movement. This combination of electromagnet and contact breaker is used in electric bells, electromagnetic hammers and electric motor horns. In all such mechanisms the movement pro duced by the magnetic action is such as to diminish the reluctance of the magnetic circuit. The magnitude of the attraction per unit area of the surface of the armature, where the normal flux density is B, is represented approximately by ir.
There are several devices in which the range of movement of the iron is greatly increased. One of these is the "coil and plunger" arrangement, in which a movable bar of iron, having one end within a coil, is drawn further within_ the coil when the current flows in either direction.
If, instead of a bar of iron, a bar of permanently magnetized steel is employed, the arrangement becomes a "polarized system," in which the movement is re versed by a reversal of the cur rent. In telephone receivers and wireless loud speakers the cur rent supplied is in the form of very feeble alternating current, and the attraction on the arma ture, being proportional to would be very small if the system were purely electromagnetic. If, however, the cores form polar ex tensions of a permanent magnet, by which they are magnetized to a suitable flux density B, the displacement of the armature due to a weak current in the coils, producing a proportional change SH in the magnetizing force, is proportional to i.e., to B.6B/6H. The displacement is thus proportional to the product of the flux density B and its rate of variation with mag netizing force 3B/8H. The value of B.SB/SH depends upon the permeability and hysteresis of the cores, since we are here dealing with small cycles of flux superposed upon the constant flux due to the permanent magnet. Materials of high permeability, such as silicon steel, are found to have the greatest values of B. SB/SH, and in some telephone receivers the cores and the diaphragm (which here forms the armature) are made of this material. The factor 8B6 H also depends upon the reluctance of the air gaps and other parts of the magnetic circuit traversed by the alternating flux, and to produce the greatest sensitiveness the reluctance must be kept as small as possible. In this respect the presence of the steel magnet in the magnetic circuit is not very favourable.
Another type of mechanism has come into use (especially in loud speakers) in which the received and amplified alternating cur rent, instead of flowing through coils fixed on the cores of a mag net, flows through a movable coil placed in the annular gap of an electromagnet of suitable form. Details of this "moving coil" system will be found in the article TELEPHONE.