One of the most interesting developments of recent years is that of "wired wireless" or "carrier-current telephony" over wires. Suppose that instead of broadcasting from the antenna at station 2 we arrange to have its radio transmitter supply current to a wire circuit. We use this same pair of wires for receiving from the distant station. We can do this if we treat the radio transmitter and receiver exactly like the telephone instruments of Fig. 132 and connect them to a square of resistances. One of these resistances is, of course, the line between the stations. I have shown the general arrangement in Fig. 134. You see what the square of resistances, or "bridge" really does for us. It lets us use a single pair of wires for messages whether they are coming or going. It does that because it lets us connect a transmitter and also a receiver to a single pair of wires in such a way that the transmitter can't affect the receiver. Whatever the transmitter sends out goes along the wires to the distant receiver but doesn't affect the receiver at the sending station. This bridge permits this whether the transmitter and receiver are radio instruments or are the ordinary telephone instruments. [Illustration: Fig 134] By its aid we may send a modulated high-frequency current over a pair of wires and receive from the same pair of wires the high-frequency current which is generated and modulated at the distant end of the line. It lets us send and receive over the same pair of wires the same sort of a modulated current as we would supply to an antenna in radio-telephone transmitting. It is the same sort of a current but it need not be anywhere near as large because we aren't broadcasting; we are sending directly to the station of the other party to our conversation. If we duplicate the apparatus we can use the same pair of wires for another telephone conversation without interfering with the first. Of course, we have to use a different frequency of alternating current for each of the two conversations. We can send these two different modulated high-frequency currents over the same pair of wires and separate them by tuning at the distant end just as well as we do in radio. I won't sketch out for you the tuned circuits by which this separation is made. It's enough to give you the idea. In that way, a single pair of wires can be used for transmitting, simultaneously and without any interference, several different telephone conversations. It takes very much less power than would radio transmission and the conversations are secret. The ordinary telephone conversation can go on at the same time without any interference with those which are being carried by the modulations in high-frequency currents. A total of five conversations over the same pair of wires is the present practice.