Quadruplex telegraphy consists in the simultaneous transmission of two messages from each end of the line. The first to attain practical success was Edison, and his method with some modifications is still the one in most general use. The arrangement is shown, and indicates the general principle involved. K1 and K2 are two transmitting keys; the former reverses the direction of the line current, the latter increases the strength irrespective of direction, by joining on another battery when the key is depressed. R1 and R2 are relays for receiving the currents; the former is polarized and responds to reversals of current, while the latter is non-polarized and responds only to the increased current from K2 irrespective of the direction of that current. This arrangement can be duplexed in the way already explained, by providing differential relays and arranging for the outgoing currents to divide differentially through the two relays at each end.