The Vacuum Impedance and Unit Systems

摘要

In the electromagnetic theory, the vacuum impedance Z_0 is a universal constant, which is as important as the velocity of light c_0 in vacuum. Unfortunately, however, its significance is not appreciated so well and sometimes the presence itself is ignored. It is partly because in the Gaussian system of units, which has widely been used for long time, Z_0 is a dimensionless constant and of unit magnitude. In this paper, we clarify that Z_0 is a fundamental parameter in electromagnetism and plays major roles in the following scenes: reorganizing the structure of the electromagnetic formula in reference to the relativity; renormalizing the quantities toward natural unit systems starting from the SI unit system; and defining the magnitudes of electromagnetic units.