The units in this system are formed in a manner similar to that of the cgs electrostatic system of units: the unit of electric current was defined using the law that describes the force between current-carrying wires. To do this, the permeability of free space (the magnetic constant, relating the magnetic flux density in a vacuum to the strength of the external magnetic field), was set at 1. To distinguish cgs electromagnetic units from units in the international system, they were often given the prefix “ab-”. However, most are often referred to purely descriptively as the 'e.m. unit of capacitance', etc.