A non-null field is characterised by P 2 + Q 2 ≠ ≠ 0 . If P ≠ ≠ 0 = Q , there exists an inertial reference frame for which either the electric or magnetic field vanishes. (These correspond respectively to magnetostatic and electrostatic fields.) If Q ≠ ≠ 0 , there exists an inertial frame in which electric and magnetic fields are proportional.