The total amount of

magnetic induction passing through a

surface, s. It is represented by the

symbol Phi, and

measured in

units of

Webers. Typically

calculated as:

Phi = ∫{ **B⋅ds** } over s.

That's:

Phi = Integral{ **B dot ds**} over s.

Here Phi is a scalar quantity, like any flux, because it measures the sum of the magnetic flux density field passing normal to the surface of s at any given point (and here **s** is a normal vector to surface s at any given point), summed over the whole surface of s.

Magnetic flux is a useful concept because it is preserved throughout a branch of a magnetic circuit in much the same way as current is preserved in an electric circuit. See Ohm's Law for Magnetics. One may change the area of the material in a magnetic circuit, but the magnetic flux flowing through the section must remain the same.