FOUCAULT CURRENTS, or EDDY CURRENTS, are electrical currents gener ated by induction within the substance of a massive conductor that is moving in a magnetic field, or which is exposed to the influence of a variable field. If the conductor is filiform, like a wire, no current can be produced in it save in the direction of its length. If a mas sive conductor be thought of, however, as com posed of an infinite number of closed circuits, each composed of a single wire and all tangled up and then melted into a solid mass, it is evi dent that (in general) currents will be in duced in all of these imaginary circuits when the conductor moves, or when the field to which it is subjected varies. In the actual case the conductor is not composed of wires melted together, but currents are nevertheless :u..nerated within it just as though these wires had an actual objective existence. Mathemat ical equations can in fact be written down, from which it is possible to compute the direc tion and intensity of the current that is flowing, at any given instant, through any proposed point of a conductor in a known but varying field. Such calculations are seldom made out side of college classrooms, however, because it is known from the general principle of the conservation of energy that the direction of the i Foucault currents is everywhere such as to oppose the change (of whatever sort it may be) which produces them. Hence such currents tend
to diminish the efficiency of all motors, dyna mos and transformers in which they occur and designers therefore strive to avoid them so far as possible. In armatures and in transformers, for example, it is customary to laminate the masses of iron that are exposed to changing fields, the individual parts being insulated from one another by air gaps or otherwise and their surfaces of separation disposed (as nearly as practicable) so as to be perpendicular to the direction in which the Foucault currents tend to flow. In galvanometers, masses of copper are often purposely disposed near the sensitive needle with distinct advantage; for while they do not affect the total deflection of the needle, they cause it to come to rest very quickly after the circuit is broken, the motion of the needle , inducing Foucault currents in the copper, which tend always to bring the needle to rest. The energy that is expended in the generation of Foucault currents is transformed into heat and raises the temperature of the mass within which the currents are flowing. The name refers to the French physicist, Jean Bernard Leon Fou cault (q.v.), who studied the subject with much care.