2. Quadrupoles and multidipoles 2.1 Definitions and conventions A network with the following properties is called a quadrupole: it is linear; it contains passive elements ; it can contain sources linked to internal quantities; it does not contain independent sources or sources linked to external quantities; it is connected to the outside only by two pairs of terminals 1,1′ and 2,2′ (figure 12); the current entering through 1 exits through 1′ and the current entering through 2 exits through 2 ′.