There is another reason, too, for keeping amateurs to the shortest wave-lengths. Transmission of radio signals over short distances is best accomplished by short wave-lengths but over long distances by the longer wave-lengths. For trans-oceanic work the very longest wave-lengths are best. The "long-haul" stations, therefore, work in the frequency range immediately above 10,000 cycles a second and transmit with wave lengths of 30,000 m. and shorter. [Illustration: 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.] LETTER 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. [Illustration: Fig 129] Imagine four wire resistances connected together to form a square as in Fig. 130. Suppose there are two pairs of equal resistances, namely _R_{1}_ and _R_{2}_, and _Z_{1}_ and _Z_{2}_. 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 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. [Illustration: Fig 130] 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_.