Already during Maxwell's lifetime, it was found that the known values for formula_19 and formula_20 give formula_21, 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 formula_22 and formula_23 are defined constants, (which means that by definition formula_24) 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.