Pl. XII.–Broadcasting Station of the American Telephone and Telegraph Company on the Roof of the Walker-Lispenard Bldg. in New York City Where the Long-distance Telephone Lines Terminate.
251LETTER 24
BY WIRE AND BY RADIO
Dear Boy:
The simplest wire telephone-circuit is formed by a transmitter, a receiver, a battery, and the connecting wire. If two persons are to carry on a conversation each must have this amount of equipment. The apparatus might be arranged as in Fig. 129. This set-up, however, requires four wires between the two stations and you know the telephone company uses only two wires. Let us find the principle upon which its system operates because it is the solution of many different problems including that of wire-to-radio connections.
Imagine four wire resistances connected together to form a square as in Fig. 130. Suppose there are two pairs of equal resistances, namely R1 and R2, and Z1 and Z2. If we connect a generator, G, between the junctions a and b there will be two separate streams of electrons, one through the R-side and the other through 252 the Z-side of the circuit. These streams, of course, will not be of the same size for the larger stream will flow through the side which offers the smaller resistance.
Half the e. m. f. between a and b is used up in sending the stream half the distance. Half is used between a and the points c and d, and the other half between c and d and the other end. It doesn’t make any difference whether we follow the stream from a to c or from a to d, it takes half the e. m. f. to keep this stream going. Points c and d, therefore, are in the same condition of being “half-way electrically” from a to b. The result is that there can be no current through any wire which we connect between c and d.