Magnetic Flux (Φ) And Magnetic Flux Density (B)

When a specimen is magnetised, lines of force or flux exist within the specimen - the stronger the magnetising force applied, the greater the amount of flux produced. The magnetising force may be applied by using a permanent magnet or electrically operated magnetic flow apparatus, or by passing an electric current through the specimen.

Magnetic flux is measured in Webers (Wb).

1 wb = 10810^8 lines of force

The number of lines of force (or flux) passing transversely through a given cross-sectional area is known as the flux density (B).

Flux density (B)=ϕA(B) = \frac{\phi}{A}
Where Φ = flux, A = Area
therefore the number of lines of flux per unit area.

Flux density is measured in Tesla (T) or Gauss

1 Wb/m2 = 1 tesla (T).

The old (cgs) unit for flux density which is still widely encountered is the gauss:
1 gauss = 1 line of force/cm2\text{force}/{\text{cm}^2}.
10,000 (104) gauss = 1 tesla.
1 gauss = 0.1 mT.

BS EN ISO 9934-1 Magnetic particle testing states a requirement of 1 tesla in the component’s surface achieved with a tangential field strength of 2 kA/m (rms). This is approximately 50% of the saturation point of most ferromagnetic steels.