If we break the current in one of the coils we immediately have magnetism in the iron; or if we destroy the balance of the two currents by making one stronger than the other we shall have magnetism of a strength that measures the difference between the two. Without specifically describing here the entire mechanism--since this is not a text-book or a treatise--we may say that a duplex telegraph-line is fitted with these differentially wound electromagnets at every station. When Station A (Fig. 3) is connected to the line by the positive pole of its battery, Station B will have its negative pole to line and its positive to earth. When A depresses his key to send a message, half the current passes by one set of coils around his differential magnet through a short resistance-coil to the earth, and the other half by the contrary coil around the magnet to the line, and so to Station B. The divided current does not affect A's own station, being neutralized by the differential magnet, but it does affect B, whose instrument responds and gives him the message. Now B may at the same time send a message to A by half of his own divided current from his own end of the line. [Illustration: Fig. 3. Represents a duplex 500-mile telegraph-line. A and B are the two terminal stations; B B', the batteries; K K', the keys; D D', the small resistance-coils, equal to the battery-resistance when the latter is not in circuit; R R', resistances each equal to the 500-mile line; and C C', condensers giving the artificial lines R R' the same capacity as the 500-mile line.] The puzzle to most people is: How can the signals pass each other in different directions on the same wire? But the signals do not have to pass each other. In effect, they pass; but in fact, it is like going round a circle--the earth forming half. A sends his message over the line to B. B sends his message to A through the earth and up A's ground-wire. The operative who is sending with positive pole to line _pushes_ his current through--so to speak--while the operative who is sending with the negative pole to line _pulls_ more current in the same direction through the line whenever he closes his key. This may not be a strictly scientific statement; but, as long as we speak of a "current" flowing from positive to negative poles (which is the invariable course electricity takes), it is the way to look at the matter understandingly. The short "resistance-coil" at each end, fortified by a "condenser" made of many leaves of isolated tin-foil, to give it capacity, offers precisely the same resistance to the current as the 500 miles of wire line; so that the twin currents that run around the differential magnet exactly neutralize each other and make no effect in the office the message starts from; while one of them takes to the earth, and the other to the line to carry the message.