OF NACSETIC 1 NDucTioN are tubes formed in the same manner in the field around magnets by drawing lines of magnetic force through the points of any small closed curve. Farattay conceived the idea of these tubes being continuous through magnets and all bodies, not ending on arty surface; they form, therefore, closed circuits, like a rubber tube with the two open ends brought together. The tubes are con ceived to be of such cross-sections that the num ber lensing a north pole of unit strength is 4r, where r 3.1416. See :MAGNETISM.
Owing to an electric current in a closed cir cuit, there is a magnetic field of force inelosing it: the tubes of magnetic induction form closed curves around the conductor. The coefficient of self-induction, or the inductance. is the number of these tubes threading through the circuit when there is a unit, current in the conductor. Sonic of these tubes of magnetic induction may also in their paths pass through :a neighboring closed conducting circuit, and the number of those tubes which do so when there is a unit current in the first circuit is called the coefficient of mutual in duction of the two circuits. It may he shown
that if there is a unit current in the second cir cuit. thus producing tubes of magnetic induction of its own, the number of these •hieh thread the first circuit is the same as in the last case. The coefficient of self-induction depends upon the shape of the circuit, the number of turns of the eondueting wire, and on the surrounding medium, and the coefficient of mutual induction depends upon these properties for inch circuit and upon their relative positions. It can be shown that inductance plays the same part in the phenom ena of electric currents that inertia or mass does in the motion of matter. The practical unit of induction is the henry (q.v.).