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 _Z_{2}_ is that of a telephone line which stretches from one telephone station to another. Suppose also that _Z_{1}_ is a telephone line exactly like _Z_{2}_ except that it doesn't go anywhere at all because it is all shut up in a little box. We'll call _Z_{1}_ 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 _Z_{2}_ and the artificial line _Z_{1}_. We can make a very good artificial line by using a condenser and a resistance. The condenser introduces something of the capacity effects which I told you were always present in a circuit formed by a pair of wires. [Illustration: Fig 131] 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 is 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. [Illustration: Fig 132] 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. [Illustration: Fig 133] 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 at 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.