Transmission over Phantom Circuits. Under proper conditions phantom circuits are better than physical circuits, and in this respect it may be noted that some long-distance operating companies instruct their operators always to give preference to phantom circuits, because of the better transmission over them. The use of phantom circuits is confined almost wholly to open-wire circuits; and while the capacity of the phantom circuit is somewhat greater than that of the physical circuit, its resistance is considerably smaller. In the actual wire the phantom loop is only half the resistance of either of the physical lines from which it is made, for it contains twice as much copper. The resistance of the repeating coils, however, is to be added.

Simplex. Simplex telegraph circuits are made from metallic circuit telephone lines, as shown in Fig. 465. The principle is identical with that of phantom telephone circuits. The potentials placed on the telephone line by the telegraph operations are equal and simultaneous. They cause no current to flow around the telephone loop, only along it. If all qualities of the loop are balanced, the telephones will not overhear the telegraph impulses. In the figure, AA are arresters, as before, GG are Morse relays; a 2-microfarad condenser is shunted around the contact of each Morse key F to quench the noises due to the sudden changes on opening the keys between dots and dashes.

Fig. 465. Simplex Telegraph Circuit
[View full size illustration.]

A simplex arrangement even more simple substitutes impedance coils for the repeating coils of Fig. 465. The operation of the Morse circuit is the same. An advantage of such a circuit, as shown in Fig. 466, is that the telephone circuit does not suffer from the two repeating-coil losses in series. A disadvantage is, that in ringing on such a line with a grounded generator, the Morse relays are caused to chatter.

Fig. 466. Simplex Telegraph Circuit
[View full size illustration.]

The circuit of Fig. 465 may be made to fit the condition of a through telephone line and a way telegraph station. The midway Morse apparatus of Fig. 467 is looped in by a combination of impedance coils and condensers. The plans of Figs. 465 and 466 here are combined, with the further idea of stopping direct and passing alternating currents, as is so well accomplished by the use of condensers.

Fig. 467. Simplex Circuit with Waystation
[View full size illustration.]