Reciprocity: Some Remarks from a Field Point of View

The principle of reciprocity finds a wide range of applications, from human relations to international trade, politics to science and engineering. Reciprocity denotes a mutual exchange between two interacting agents. Technically, it describes the reversibility or bilateralism of an interaction upon interchange of the source and target. Although a loose statement of the principle is usually sufficient for many cases, a precise definition is essential in science and engineering. This short tutorial aims to discuss the reciprocity principle in electromagnetic engineering.