The electromagnetism exists naturally in the environment: static electricity, magnetising phenomena, spark.... The corresponding phenomena could be described by the partial differential equations - the equations of Maxwell - that often could be solved with difficulties, because there are necessary numeric methods (as finite elements methods).
The artificial electric devices, are designed by the persons and their comportment is much more simple. Its can be, generally, characterised by electric currents that flow in their terminals and differences between the potentials the terminals.
Figure 1: Multi-pole (n-poles)
The difference between the potentials (or the voltage)
between the terminal 
and the terminal 
is measured with a voltmeter. It is expressed in volts (symbol: V). The current 
that goes in (or flows out, depending on the chosen signs convention) the terminal 
is measured with an ampermeter. It is expressed in amperes (symbol: A).
In this context, the most simple electric components are the dipoles. The most part of the multi-poles can be decomposed in elementary dipoles.
One dipole can be characterised by the current that flows through it and the voltage 
across it's terminals.
