We have seen that a magnetic needle is deflected by a current passing through a coil of wire placed round it, and that the direction in which the needle is deflected depends upon the direction of the current in the coil. Now suppose we place round the needle two coils of wire, wound so that the current in one flows in a direction opposite to that of the current in the other. Then, if we pass two equal currents, one through each coil, it is evident that they will neutralize one another, so that the needle will not be deflected at all. In a duplex system one end of one of these coils is connected to earth, say to a copper plate buried in the ground, and one end of the other to the line wire. The two remaining ends are arranged as branches leading off from a single wire connected with the transmitting key. The whole arrangement of coils and needle is repeated at the other end of the line. If now the transmitting key at station A is pressed, the circuit is closed and a current flows along the single wire, and then divides into two where the wire branches, half of it taking the path through one coil and half the path through the other. Equal currents thus flow through the oppositely wound coils, and the needle at station A is not deflected. Leaving the coils, one of these equal currents flows away to earth, while the other passes out along the line wire. On its arrival at station B the current is able to pass through only one of the coils round the needle, and consequently the needle is deflected and the signal given. In this way the transmitting operator at station A is able to signal to station B without affecting the receiver at his own end, and similarly the operator at station B can transmit to A without affecting the B receiver. Thus there can be no confusion whether the transmitters are worked at different times or simultaneously, for each transmitter affects only the receiver at the opposite end of the line. The diagram in Fig. 30 will help to make clearer the general principle. K and K¹ are the two transmitting keys which close the circuit, and C and C¹ are the points at which the current divides into two. Instead of coils and needles, electro-magnets operating sounders may be used, such magnets having two separate and oppositely wound coils, acting in exactly the same way as the coils round the needles. The above description is of course only a rough outline of the method, and in practice matters are more complicated, owing to the necessity for carefully adjusted resistances and for condensers. There is also another and different method of duplexing a line, but we have not space to describe it. Duplex telegraphy requires two operators at each end of the line, one to send and the other to receive.