a series of magnetic materials through which a magnetic flux passes. The concept is extensively used in the design of electrical machines, transformers, permanent magnets, electromagnets, relays, magnetic amplifiers, and electrical measuring instruments. In technology extensive use is made of magnetic circuits in which virtually the entire magnetic flux passes through ferromagnetic bodies (closed magnetic circuits), as well as circuits that include diamagnetic mediums, such as air gaps, along with the ferromagnetic materials. If the flux in a magnetic circuit is excited by permanent magnets, the circuit is called a polarized circuit. Circuits without permanent magnets are called neutral, and the magnetic flux in them is excited by a current flowing through windings that surround part or all of the magnetic circuit.

A distinction is made among magnetic circuits with constant, alternating, and pulsed magnetic fluxes, depending on the nature of the excitation current. As a result of the complete formal analogy between electrical and magnetic circuits, they have a common mathematical treatment. For example, the analogue of Ohm’s law for a magnetic circuit is the formula F = Φ.R_m, where Φ is the magnetic flux, R_m is the magnetic resistance, and Fis the magnetomotive force. Kirchhoff’s laws are applicable to magnetic circuits. However, there is a major difference between the two types of circuits. A flux Φ in a magnetic circuit that does not vary with time produces no heating effect—that is, no electromagnetic energy is dissipated.