unity, is called the c.g.s. unit of magnetism in the electrodynamic system.
We have thus two different systems of measurement, the electrostatic and the electrodynamic. They have been adopted as a matter of convenience in order to be able to regard in both the ethereal coefficient of air as unity. The result of this is that the absolute magnitude of electric quantity in the two systems is very different. In the electrostatic system unit quantity is exceedingly small as compared to the amount of electricity which goes to make up one unit of charge in the electrodynamic system. It requires 30,000 millions electrostatic units to make up one electrodynamic (or, as it is also called, electromagnetic) unit of electricity. The speed of light is 30,000 millions cm. per second. It is highly improbable that the agreement between the speed of light and the numerical ratio between the units should be a mere coincidence; but if it is not, then the ratio between the units is not merely a numeric but something which has a particular character, namely, the character of velocity, that is, a length divided by a time. Further, if we rule out the idea of a merely accidental agreement between two numbers, we are
