viding transmission of such a grade as would require from 5 to 10
times as much copper in the cable circuits if loading were not
employed. Loading coils have been materially improved by
constructing the cores of several rings, each of which is made by
compressing finely divided particles of iron with a binding material which
acts as insulation between the iron particles. There may be as many
as thirty thousand million of these particles in the core of a cable
loading coil. These cores are more uniform and stable than the wire
cores formerly used and are much less affected by excessive currents
which may accidentally come into the circuit.
British Submarine Telephone Cables.
| Particulars of Cable | Length | Weight per naut. m. | Loop Resistance per naut. m. of Conductor without loading (at 60° F.) |
Alternating Current Constants | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Conductor | Dielectric | Circuit | Frequency of Test |
Added Resistance per naut. m. due to Loading |
Inductance (L) per naut. m. |
Capacity (C) per naut. m. |
Leakance Capacity or G/C. |
Attenuation Constant per naut. m. |
Characteristic Impedance (Zo) | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Naut. m. | Lb. | Lb. | Ohms. | ∼ Per sec. | Ohms. | Henries | μF | Ohms. | |||||||||||||||||||||||||||||||||||||||||||||||||||||
|
47.9 | 160 | 150 | 14.3 |
|
|
|
|
|
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||
|
63.3 | 160 | 150 | 14.3 |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
22.0 | 169 | 195 | 13.5 |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
21.0 | 160 | 300 | 14.3 | Physical | 750 | 6.0 | 0.095 | 0.186 | 120 | 0.0148 | 1,000\3° 43′ | |||||||||||||||||||||||||||||||||||||||||||||||||
|
21.0 | 300 | 300 | 7.6 |
|
1,000 1,000 |
— — |
0.0135 — |
0.176 — |
109 — |
0.0185 0.0185 |
278 \2° 59′ — | |||||||||||||||||||||||||||||||||||||||||||||||||
|
|
310 | 200 | 7.4 |
|
800 800 |
5.2 2.5 |
0.080 0.040 |
0.189 0.378 |
20 20 |
0.0112 0.0114 |
709 \1° 50′ 338 \1° 40′ | |||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
160 | 150 | 14.3 |
|
800 800 |
6.2 3.0 |
0.100 0.050 |
0.166 0.320 |
20 20 |
0.0145 0.0140 |
776 \1° 3′ 395 \1° 2′ | |||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Long-Distance Telephony Open Wire.—At the beginning of the decade 1910-20, the limits of telephone transmission were about 1,200 to 1,500 m. in open wire. These limits were extended rapidly so that in 1921 practically all parts of the continental United States were placed in communication with each other over distances of 4,000 m. and upwards, employing overhead wires no larger than those used to give the restricted service of 1910. These improvements were made with only slight changes in the lines and equipment and with no change whatever in the subscriber's station apparatus. They depended upon the development of satisfactory repeaters with their associated apparatus and methods of use. The form of repeater generally employed in 1921 was the 3-element thermionic tube. Devised primarily for radio purposes, it was so adapted as to become a remarkably effective repeater. This required that a large amount of auxiliary apparatus be invented and developed and methods devised for balancing the lines and making them suitable for the operation of this apparatus. The amplifier or repeater receives the minute attenuated telephone currents and sends out currents of exactly the same form but greatly enlarged. The transmission gain which may be obtained with vacuum-tube amplifiers in two-way operation depends on the electrical conditions of the line in which the amplifiers are used. This has a great effect on line design.
Transcontinental Telephony.—By the development of methods by which the loading coil could be applied to the heaviest gauge wires and such wires, when equipped with loading coils, could be operated on the phantom principle, it became practicable, in 1911, to provide telephone service between New York City and Denver, Col., and greatly to improve the transmission of speech between cities less far apart. By the application of the phantom principle to such circuits the available facilities were largely increased so that, between the important telephone centres, notable improvements in service were accomplished. On Jan. 25 1915, the transcontinental line of the Bell System was formally opened for business and after that timc,commercial service was given between the cities on the Atlantic Coast and those on the Pacific Coast. The service in 1921 was handled over a group of 4 non-loaded wires equipped with telephone repeaters. By using the 2 side circuits and the phantom circuit formed by these wires, 3 simultaneous transcontinental connexions may be established. By means of the addition of compositing apparatus to the circuits the 4 wires which carry 3 telephone circuits also carry 4 telegraph circuits. These 4 telegraph circuits may be arranged to transmit 8 simultaneous messages. The line from New York City to San Francisco is 3,400 m. in length.
Long-Distance Telephony Cables.—By 1906 a cable 90 m. long was successfully operated between New York and Philadelphia, but, in the then state of the art, that cable could not be used for connexions extending beyond New York or Philadelphia. In 1911, an underground cable was designed capable of giving a satisfactory conversation between Washington and Boston. By 1912, a section of this new cable was laid from Washington to Philadelphia, there connecting with the earlier type of cable to New York. During 1913, a section of the new cable was laid between New Haven and Providence, connecting at New Haven with an earlier type of cable extending to New York and connecting at Providence with an earlier type extending to Boston. Although talking over the whole distance from Boston to Washington was not possible so long as stretches of cable of the older types had to be employed, yet by using the underground in connexion with the overhead, the seaboard cities from Washington to Boston could no longer be isolated by storms destroying the overhead lines. During 1913, the advances in the art of loading and balancing underground circuits together with the repeater developments made it possible to talk satisfactorily by underground wires from Boston to Washington, a distance of 455 m. even though 47% of the total cable in the line was of the types formerly suitable for short-haul working only. In 1912, talking by underground wire for the first time between New York and Washington represented the longest distance achieved. By 1913, this distance had been doubled. The Boston-Washington cable was several times longer than any other in the world. There were in 1921 several cables working along the Boston-Washington route. During 1919, the extension of the toll cable system from Philadelphia to Harrisburg, Pa., was completed. Taken in combination with the cables already working between Boston and Washington, this gave a through toll cable route from the important points on the eastern seaboard as far west as Harrisburg. In 1921, this cable was extended from Harrisburg as far W. as Pittsburgh, a distance of 192 m. from Harrisburg and 304 m. from Philadelphia. For the greater portion of the distance the cable was supported aerially on poles. The composition of this cable was as follows:—
| Section | Quads No. 16 A.W.G. | Quads No. 19 A.W.G. | |
| Harrisburg-Ligonier | 16 | 125 | |
| Ligonier-Pittsburgh | 19 | 120 |
