1. General

The currents of interest in this article can be either continuous or alternating. By direct current we mean a current whose value remains constant over time, or whose variations are negligible during the measurement period. In this case, the quantity to be measured is perfectly defined. An alternating current, on the other hand, is a current whose value varies at each instant, and whose direction also changes over time. It is, however, periodic, i.e. it returns to identical values at regular time intervals. This last property means that there are parameters, remaining constant over time, which can characterize such a current in terms of amplitude. These are the parameters we measure. In addition to being periodic, an alternating current has the property of having a zero mean value.