In physics, specifically electromagnetism, the magnetic flux (often denoted F or F_B) through a surface is the component of the B field passing through that surface. The SI unit of magnetic flux is the weber (Wb) (in derived units: volt-seconds), and the CGS unit is the maxwell. Magnetic flux is usually measured with a fluxmeter, which contains measuring coils and electronics that evaluates the change of voltage in the measuring coils to calculate the magnetic flux.