In a frame moving at velocity v, the E -field in the moving frame when there is no E -field in the stationary magnet frame Maxwell's equations transform as: E ′ = γ γ v × × B where γ γ = 1 1 − − ( v / c ) 2 is called the Lorentz factor and c is the speed of light in free space. This result is a consequence of requiring that observers in all inertial frames arrive at the same form for Maxwell's equations. In particular, all observers must see the same speed of light c. That requirement leads to the Lorentz transformation for space and time. Assuming a Lorentz transformation, invariance of Maxwell's equations then leads to the above transformation of the fields for this example.