Abstract :
This paper attempts to present more clearly the principles underlying the definition of a system of electrical units. First, it recalls briefly how fundamental and derived quantities are defined and how fundamental quantities are chosen. The main laws of electromagnetism are then written in a general form using the least number of arbitrary constants, namely three, compatible with the assumption that all the electrical units are derived in a consistent manner from the units of charge and current. It is shown how a system of units is selected and how this choice determines the values and dimensions of the three constants, two of which are just ¿o and ¿o. These considerations lead us to a discussion of the more or less fundamental nature of the four vectors describing the electromagnetic field and of the different ways in which these vectors can be associated.
