Volume Ii Part 22 (1/2)

=Definitions.= Phantom circuits are arrangements of telephone wires whereby more working, non-interfering telephone lines exist than there are sets of actual wires. When four wires are arranged to provide three metallic circuits for telephone purposes, two of the lines are physical circuits and one is a phantom circuit.

Simplex and composite circuits are arrangements of wires whereby telephony and telegraphy can take place at the same time over the same wires without interference.

[Ill.u.s.tration: Fig. 460. Phantom Circuit]

=Phantom.= In Fig. 460 four wires join two offices. _RR_ are repeating coils, designed for efficient transforming of both talking and ringing currents. The devices marked _A_ in this and the following figures are air-gap arresters. Currents from the telephones connected to either physical pair of wires pa.s.s, at any instant, in opposite directions in the two wires of the pair. The phantom circuit uses one of the physical pairs as a _wire_ of its line. It does this by tapping the middle point of the line side of each of the repeating coils. The impedance of the repeating-coil winding is lowered because, all the windings being on the same core, the phantom line currents pa.s.s from the middle to the outer connections so as to neutralize each other's influence. The currents of the phantom circuit, unlike those of the physical circuits, are _in the same direction_ in both wires of a pair at any instant.

Their potentials, therefore, are equal and simultaneous.

A phantom circuit is formed most simply when both physical lines end in the same two offices. If one physical line is longer than the other, a phantom circuit may be formed as in Fig. 461, wherein the repeating coil is inserted in the longer line where it pa.s.ses through a terminal station of the shorter.

[Ill.u.s.tration: Fig. 461. Phantom from Two Physical Circuits of Unequal Length]

[Ill.u.s.tration: Fig. 463. Two Phantoms Joined by Physical Circuit]

A circuit may be built up by adding a physical circuit to a phantom. A circuit may be made up of two or more phantom circuits, joined by physical ones. In Fig. 462 a phantom circuit is extended by the use of a physical circuit, while in Fig. 463, two phantom circuits are joined by placing between them a physical circuit.

[Ill.u.s.tration: Fig. 462. Phantom Extended by Physical Circuit]

_Transpositions._ In phantom circuits formed merely by inserting repeating coils in physical circuits and doing nothing else, an exact balance of the sides of the phantom circuit is lacking. The resistances, insulations, and capacities to earth of the sides may be equal, but the exposures to adjacent telephone and telegraph circuits and to power circuits will not be equal unless the phantom circuits are transposed.

To transpose a set of lines of two physical wires each, is not complicated, though it must be done with care and in accordance with a definite, foreknown plan. Transposing phantom circuits is less simple, however, as four wires per circuit have to be transposed, instead of two.

[Ill.u.s.tration: Fig. 464. Transposition of Phantom Circuits]

In Fig. 464, the general s.p.a.cing of transposition sections is the usual one, 1,300 feet, of the _ABCB_ system widely in use. The pole circuit, on pins _5_ and _6_ of the upper arm, is transposed once each two miles.

The pole circuit of the second arm transposes either once or twice a mile. But neither pole circuit differs in transposition from any other regular scheme except in the frequency of transposition. All the other wires of each arm, however, are so arranged that each wire on either side of the pole circuit moves from pin to pin at section-ends, till it has completed a cycle of changes over all four of the pins on its side.

In doing so, each phantom circuit is transposed with proper regard to each of the other three on that twenty-wire line.

The ”new transposition” lettering in Fig. 464 is for the purpose of identifying the exact scheme of wiring each transposition pole. The complication of wiring at each transposition pole is increased by the adoption of phantom circuits. Maintenance of all the circuits is made more costly and less easy unless the work at points of transposition is done with care and skill. Phantom circuits, to be always successful, require that the physical circuits be balanced and kept so.

_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.

[Ill.u.s.tration: Fig. 465. Simplex Telegraph Circuit]

A simplex arrangement even more simple subst.i.tutes 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.

[Ill.u.s.tration: Fig. 466. Simplex Telegraph Circuit]

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 pa.s.sing alternating currents, as is so well accomplished by the use of condensers.

[Ill.u.s.tration: Fig. 467. Simplex Circuit with Waystation]

[Ill.u.s.tration: Fig. 468. Composite Circuit]

=Composite.= Composite circuits depend on another principle than that of producing equal and simultaneous potentials on the two wires of the telephone loop. The opposition of impedance coils to alternating currents and of condensers to direct currents are the fundamentals. The early work in this art was done by Van Rysselberghe, of Belgium. In Fig.

468, one telephone circuit forms two Morse circuits, two wires carrying three services. Each Morse circuit will be seen to include, serially, two 50-ohm impedance coils, and to have shunts through condensers to ground. The 50-ohm coils are connected differentially, offering low consequent impedance to Morse impulses, whose frequency of interruption is not great. As the impedance coils are large, have cores of considerable length, and are wound with two separate though serially connected windings each, their impedance to voice currents is great.