When alternating current flows in a conductor it radiates an electromagnetic wave (radio wave). In multi-element antennas, the fields due to currents in one element induce currents in the other elements. Antennas are self-interacting in this respect; the waves reradiated by the elements superimpose on the original radio signal being studied. NEC calculates the field resulting from these contributions, adds it to the original radio signal, and then runs the entire calculation again with this modified field. Because the reradiated signal is normally small compared to the original signal, it only produces a small change, or perturbation, in the resulting element currents. The program then repeats the calculation again with the new element currents, getting new radiation fields. This process is repeated until resulting values converge.