Already during Maxwell's lifetime, it was found that the known values for ε ε 0 and μ μ 0 give c ≈ ≈ 2.998 × × 10 8 m/s , then already known to be the speed of light in free space. This led him to propose that light and radio waves were propagating electromagnetic waves, since amply confirmed. In the old SI system of units, the values of μ μ 0 = 4 π π × × 10 − − 7 and c = 299 792 458 m/s are defined constants, (which means that by definition ε ε 0 = 8.854 187 8... × × 10 − − 12 F/m ) that define the ampere and the metre. In the new SI system, only c keeps its defined value, and the electron charge gets a defined value.