where c is the speed of light in vacuum (e "" 3 x 108 m /sec).
This relationship between the speed of light and a physical constant was an important result of the early electromagnetic theory in the late nineteenth century, and showed that light is an electromagnetic wave; see the discussion in Chapter 7.
To obtain a feel of how large the force in (I) is, we compare it with the gravitational force that is also an inverse square law with distance. The smallest unit of charge known is that of an electron with charge t and mass m.