Induction

Induction, in statical electricity, signifies the appearance of electricity of one kind on the surface of separation of a dielectric and a conducting substance, when electricity of the opposite kind has been isolated. It is impossible to produce one without the other ; when a charged body is introduced into a room, the inner walls have the opposite charge distributed over them, the density of the induced charge at any point depending upon the shape of the charged body, the intensity of its charge, its distance from the enclosure, and the shape of the enclosure. The dielectric between these opposite charges is in a stressed condition, for their tendency is to combine. When a rubbed piece of ebonite is held near a few shreds of light paper, the charge in the ebonite causes the induced charge in the neighbouring substances to be practically concentrated in the nearest of these, i.e. in the shreds of paper. The tendency of the opposite charges to unite will cause the paper to attach itself to the ebonite and so discharge itself. It then falls, the residual charge on the ebonite causes a further flow of electricity into the paper, and the process is repeated though with less vigour.

In magnetism the term has a somewhat different significance. The medium surrounding a magnet is in a strained condition, and at every point there is a definite direction of the resultant force; the existence of this force is not apparent unless a magnetic substance, such as iron, be brought to the given point. When any such substance is thus introduced into the medium, the lines of force are altered in direction, and the intensity of the force is changed. This rearrangement of the condition of the medium by reason of the introduction of a magnetic substance is called magnetic induction. [Magnetism.] In electro-magnetism it is found that variation in the arrangement of the lines of induction in the medium surrounding a magnet is capable of producing electric currents in any closed conducting circuits, through which the varying lines of induction may pass. If, for example, a bar magnet be thrust through a closed coil of wire, a current will be found to pass round the coil. The strength of this induced current depends on the number of windings in the coil, their diameter, and their resistance, on the strength of the inserted magnet, and the time taken to thrust it into the coil. Inasmuch as lines of induction may be produced by an electric circuit precisely similar to those produced by a magnet, it follows that variation in the strength of current flowing in one circuit will cause intermittent currents to exhibit themselves in any neighbouring circuit. This fact has wide-reaching applications in modern electrical engineering. [Dynamo-Electrical Machinery.]

The Ruhmltorff Induction, Coil exhibits the aption of one current on another very clearly. A low-resistance coil of few turns is wound round a core of soft iron, generally made up of a bundle of fine iron wires. Surrounding this, but entirely insulated electrically, is another coil of wire finer than the first, of many thousands of turns and much higher resistance in consequence. A current obtained from a few cells is sent through the low resistance or primary coil for a small fraction of a second and then stopped. This starting and stopping is rendered continuous by a trembler mechanism similar to that used in ordinary trembler electric bells (q.v.) for the same purpose. This variation in the current induces an alternation of potential in the high resistance or secondary circuit, in which the actual current strength will average much less than in the primary, but the intensity of potential much greater. [Vacuum Tubes.]