The tangent law of magnetism is a way of measuring the strengths of two perpendicular magnetic fields. When a magnet is exposed to a magnetic field B that is perpendicular to the Earth’s horizontal magnetic field (Bh), the magnetic field will rest at an angle theta. The relation between the magnetic fields is as follows:
B = Bh tanθ or
tanθ = B/Bh
The law is known as the tangent law of magnetism because tangent of an angle is the relevant factor here.