=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.
They act as though they were not connected differentially, so far as voice currents are concerned.
Because of the condensers serially in the telephone line, voice currents can pa.s.s through it, but direct currents can not. Impulses due to discharges of cores, coils, and capacities in the Morse circuit _could_ make sounds in the telephones, but these are choked out, or led to earth by the 30-ohm impedance coils and the heavy Morse condensers.
=Ringing.= Ringing over simplex circuits is done in the way usual where no telegraph service is added. Both telegraphy and telephony over simplex circuits follow their usual practice in the way of calling and conversing. In composite working, however, ringing by usual methods either is impossible because of heavy grounds and shunts, or if it is possible to get ringing signals through at all, the relays of the Morse apparatus will chatter, interfering with the proper use of the telegraph portion of the service.
It is customary, therefore, either to equip composite circuits with special signaling devices by which high-frequency currents pa.s.s over the telephone circuits, operating relays which in turn operate local ringing signals; or to refrain from ringing on composite circuits and to transmit orders for connections by telegraph. The latter is wholly satisfactory over composite lines between points having heavy telegraph traffic, and it is between such points as these that composite practice is most general.
=Phantoms from Simplex and Composite Circuits.= Phantom and simplex principles are identical, and by adding the composite principle, two simplex circuits may have a phantom superadded, as in Fig. 469.
Similarly, as in Fig. 470, two composite circuits can be phantomed. This case gives seven distinct services over four wires: three telephone loops--two physical and one phantom--and four Morse lines.
[Ill.u.s.tration: Fig. 469. Phantom of Two Simplex Circuits]
[Ill.u.s.tration: Fig. 470. Phantom of Two Composite Circuits]
=Railway Composite.= The foregoing are problems of making telegraphy a by-product of telephony. With so many telegraph wires on poles over the country, it has seemed a pity not to turn the thing around and provide for telephony as a by-product of telegraphy. This has been accomplished, and the result is called a railway composite system. For the reason that the telegraph circuits are not in pairs, accurately matched one wire against another, and are not always uniform as to material, it has not been possible to secure as good telephone circuits from telegraph wires as telegraph circuits from telephone wires.
Practical results are secured by adaptation of the original principle of different frequencies. A study of Fig. 468 shows that over such a composite circuit the usual method of ringing from station to station over the telephone circuit by an alternating current of a frequency of about sixteen per second is practically impossible. This is because of the heavy short-circuit provided by the two 30-ohm choke coils at each of the stations, the heavy shunt of the large condensers, and the grounding through the 50-ohm choke coils. If high-frequency speech currents can pa.s.s over these circuits with a very small loss, other high-frequency circuits should find a good path. There are many easy ways of making such currents, but formerly none very simple for receiving them. Fig. 471 shows one simple observer of such high-frequency currents, it being merely an adaptation of the familiar polarized ringer used in every subscriber"s telephone. In either position of the armature it makes contact with one or the other of two studs connected to the battery, so that in all times of rest the relay _A_ is energized. When a high-frequency current pa.s.ses through this polarized relay, however, there is enough time in which the armature is out of contact with either stud to reduce the total energy through the relay _A_ and allow its armature to fall away, ringing a vibrating bell or giving some other signal.
[Ill.u.s.tration: Fig. 471. Ringing Device for Composite Circuits]
Fig. 472 shows a form of apparatus for producing the high-frequency current necessary for signaling. It is evident that if a magneto generator, such as is used in ordinary magneto telephones, could be made to drive its armature fast enough, it also might furnish the high-frequency current necessary for signaling through condensers and past heavy impedances.
[Ill.u.s.tration: Fig. 472. Ringing Current Device]
Applying these principles of high-frequency signals sent and received to a single-wire telegraph circuit, the arrangement shown in Fig. 473 results, this being a type of railway composite circuit. The princ.i.p.al points of interest herein are the insertion of impedances in series with the telegraph lines, the shunting of the telegraph relays by small condensers, the further shunting of the whole telegraph mechanism of a station by another condenser, and thus keeping out of the line circuit changes in current values which would be heard in the telephones if violent, and might be inaudible if otherwise.
[Ill.u.s.tration: Fig. 473. Railway Composite Circuit]
[Ill.u.s.tration: FRONT OF LONG-DISTANCE POWER BOARD U.S. Telephone Company, Cleveland, Ohio. _The Dean Electric Co._]
A further interesting element is the very heavy shunting of the telephone receiver by means of an inductive coil. This shunt is applied for by-path purposes so that heavy disturbing currents may be kept out of the receiver while a sufficient amount of voice current is diverted through the receiver. It is well to have the inductance of this shunt made adjustable by providing a movable iron core for the shunt winding.
When the core is drawn out of the coil, its impedance is diminished because the inductance is diminished. This reduces the amount of disturbing noise in the receiver. The core should be withdrawn as little as the amount of disturbance permits, as this also diminishes the loudness of the received speech.
Because the signaling over lines equipped with this form of composite working results in the ringing of a bell by means of local current, it is of particular advantage in cases where the bell needs to ring loudly.
Switch stations, crossings, and similar places where the attendant is not constantly near the telephone can be equipped with this type of composite apparatus and it so offers a valuable subst.i.tute for regular railway telegraph equipment, with which the attendant may not be familiar. The success of the local bell-ringing arrangement, however, depends on accurate relay adjustment and on the maintenance of a primary battery. The drain on the ringing battery is greater than on the talking battery.
A good subst.i.tute for the bell signal on railway composite circuits is a telephone receiver responding directly to high-frequency currents over the line. The receiver is designed specially for the purpose and is known as a "howler." Its signal can be easily heard through a large room. The condenser in series with it is of small capacity, limiting the drain upon the line. Usually the howler is detached by the switch hook during conversation from a station.
_Railway Composite Set._ The circuit of a set utilizing such an arrangement together with other details of a complete railway composite set is shown in Fig. 474. The drawing is arranged thus, in the hope of simplifying the understanding of its principles. It will be seen that the induction coil serves as an interrupter as well as for transmission.
All of the contacts are shown in the position they have during conversation. The letters _Hc1_, _Hc2_, etc., and _Kc1_, _Kc2_, etc., refer to hook contacts and key contacts, respectively, of the numbers given. The arrangements of the hook and key springs are shown at the right of the figure. _RR_ represent impedance coils connected serially in the line and placed at terminal stations. The composite telephone sets are bridged from the line to ground at any points between the terminal impedance coils.
The direct currents of telegraphy are prevented from pa.s.sing to ground through the telephone set during conversation by the 2-microfarad condenser which is in series with the receiver. They are prevented from pa.s.sing to ground through the telephone set when the receiver is on the hook by a .05 microfarad condenser in series with the howler. The alternating currents of speech and interrupter signaling are kept from pa.s.sing to ground at terminals by the impedance coils.
Signals are sent from the set by pressing the key _K_. This operates the vibrator by closing contacts _Kc6_ and _Kc7_. The howler is cut off and the receiver is short-circuited by the same operation of the key. The impedance of the coil _I_ is changed by moving its adjustable core.
[Ill.u.s.tration: Fig. 474. Railway Composite Set]
=Applications.= A chief use of composite and simplex circuits is for ticket wire purposes. These are circuits over which long-distance operators instruct each other as to connecting and disconnecting lines, the routing of calls, and the making of appointments. One such wire will care for all the business of many long-distance trunks. The public also absorbs the telegraph product of telephone lines. Such telegraph service is leased to brokers, manufacturers, merchants, and newspapers. Railway companies use portable telephone adjuncts to telegraph circuits on trains for service from stations not able to support telegraph attendants, and in a limited degree for the dispatching of trains.