Suppose, therefore, that we connect a telephone receiver between c and d. No current flows in it and no sound is emitted by it. Now suppose the resistance of Z2 is that of a telephone line which stretches from one telephone station to another. Suppose also that Z1 is a telephone line exactly like Z2 except that it doesn’t go anywhere at all because it is all shut up in a little box. We’ll call Z1 an artificial telephone line. We ought to call it, as little children would say, a “make-believe” telephone line. It doesn’t fool us but it does fool the electrons for they can’t tell the difference between the real line Z2 and the artificial line Z1. We can make a very good artificial line by using a condenser and a resistance. The condenser introduces something of the capacity effects 253 which I told you were always present in a circuit formed by a pair of wires.
At the other telephone station let us duplicate this apparatus, using the same real line in both cases. Instead of just any generator of an alternating e. m. f. let us use a telephone transmitter. We connect the transmitter through a transformer. The system then looks like that of Fig. 131. When some one talks at station 1 there is no current through his receiver because it is connected to c and d, while the e. m. f. of the transmitter is applied to a and b. The transmitter sets up two electron streams between a and b, and the stream which flows through the Z-side of the square goes out to station 2. At this station the electrons have three paths between d and b. I have marked these by arrows and you see that one of them is through the receiver. The current which is started by the transmitter at station 1 will therefore operate the receiver at station 2 but not at its own station. Of course station 2 can talk to 1 in the same way.
The actual set-up used by the telephone company 254is a little different from that which I have shown because it uses a single common battery at a central office between two subscribers. The general principle, however, is the same.
It won’t make any difference if we use equal inductance coils, instead of the R-resistances, and connect the transmitter to them inductively as shown in Fig. 132. So far as that is concerned we can also use a transformer between the receiver and the points c and d, as shown in the same figure.
We are now ready to put in radio equipment at station 2. In place of the telephone receiver at station 2 we connect a radio transmitter. Then whatever a person at station 1 says goes by wire to 2 and on out by radio. In place of the telephone transmitter 255at station 2 we connect a radio receiver. Whatever that receives by radio is detected and goes by wire to the listener at station 1. In Fig. 133 I have shown the equipment of station 2. There you have the connections for wire to radio and vice versa.
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.