Page:EB1922 - Volume 32.djvu/736

latter. The effect was to reduce materially the attenuation constant and increase the range of speech in loaded cables.

The improved dielectric was used in a cable laid in Aug. 1912 between St. Margaret's Bay, Dover, and La Panne, Belgium. This cable contained four copper conductors, each weighing 160 lb. per nautical mile and insulated by a dielectric weighing 150 lb. per mile (as compared with 300 lb. per mile in the 1910 Anglo-French cable). The variation of attenuation with frequency is much less in the 1912 cable than in the earlier one.

The Anglo-Belgian cable had another special feature, namely, the provision of loading coils for a third circuit superposed on the two physical circuits. The loading coils for all three circuits were placed together at intervals of one nautical mile.

A similar cable with some further improvements in dielectric and loading coils was laid across the Irish Sea between Nevin, Carnarvonshire, and Howth, co. Dublin, in 1913.

A submarine telephone cable of the continuously loaded type was laid across the English Channel by the French Government in 1912, between the same points as the 1910 coil-loaded cable. The weight per nautical mile of dielectric is the same in both cables, but each copper conductor of the former weighs 300 lb. per mile as compared with 160 lb. per mile in the latter. The transmission efficiency of the cables is practically equal, but the continuously loaded cable provides an additional circuit by superposing. Experiments conducted on this cable in 1914 proved the possibility of obtaining four circuits from a continuously loaded 4-wire submarine cable by introducing an improved method of balancing the electrostatic capacity of the conductors. The fourth circuit has not yet been successful in a coil-loaded cable.

Several additional coil-loaded telephone cables were laid across the English Channel during the war period. Details of these cables are given in the Table.

With equal weights of conductor and dielectric, the relative transmission efficiencies of (a) coil-loaded and (b) continuously loaded 4-wire submarine cables are as 100 to 75, but the latter may provide four circuits as compared with three in the former. Experience has shown that the maintenance and repairs of coil-loaded cables are attended by difficulties which are not met with in continuously loaded cables.

The introduction of telephone relays has made it possible frequently to use submarine cables of a less efficiency than the coil-loaded cables previously required. Consequently, it is practicable to increase the use of continuously loaded cables, and the modern tendency is in that direction.

Particulars of Representative Types of Loaded Main Underground British Cables.

Cable

 Length 

Number
 of Pairs 

Weight per
 Mile Single 
Conductor

D. C. Constants of
Cable per Mile
Loop

A. C. Constants of Loaded Cable per
Mile Loop at ω = 5,000

Inductance
 of Loading 
Coils

Average
 Distance 
between
Loading
Coils

---

---

 Resistance, 
R

 Capacity 
Wire to
Wire, C

 Inductance, 
L

Attenuation
 Constant, β 

 Characteristic 
Impedance,
Z_o

---

---

---

---

---

---

---

---

---

---

---

Miles

Ohms

μF

Henries

Henries

Miles

 Leeds-Hull

  58.6

$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	48
	6

	70
	100	*
$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	100
	100	†
	150
	200
	300
	150	phantom
	100	phantom
$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	100
	100	†
	150
	200
	300
	150	phantom
	100	phantom

28.7 

0.065 

0.052 

0.0166 

  897 \3°  4′

 0.133 

 2.55

—

—

—

—

—

—

London-	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
Birmingham

 109.5

$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	24
	12
	14
	2
	6
	6

18.95

0.0575

0.0535

0.01092

  905 /5° 41′

	0.133	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$
	0.133
	0.133
	0.133
	0.133
	0.0825
	0.0825

 2.5

18.38

0.0568

0.053 

0.01074

  891 /5° 36′

13.13

0.0697

0.0537

0.00884

  864 /8° 40′

 9.62

0.0654

0.0536

0.00664

  860 /8°  7′

 6.55

0.0567

0.0547

0.00408

  959/13°  7′

 6.56

0.1056

0.0345

0.00756

  563 /6° 48′

 9.5 

0.0905

0.0357

0.00926

  610 /8° 55′

Birmingham-	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
Liverpool

  89.9

$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	24
	12
	14
	2
	6
	6

17.9 

0.0579

0.053 

0.01057

1 062 \1°  4′

	0.133	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$
	0.133
	0.133
	0.133
	0.133
	0.0825
	0.0825

 2.5

17.32

0.0575

0.053 

0.01014

1 069 \2° 57′

12.44

0.0685

0.0537

0.00846

1 031 \1°  4′

 9.19

0.0572

0.0536

0.00618

1 088 \5° 45′

 6.28

0.0545

0.055 

0.00413

1 074 /2° 26′

 6.22

 0.10075

0.0345

0.00682

  565 /1° 18′

 8.95

0.0864

0.0357

0.00837

  615 \2° 11′

 D. C. Constants of 
Unloaded Cables

---

R

C

London-	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
Manchester

 186.5

160 

$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	40
	40	phantom

44.00 

0.065 

0.109 

0.0208 

1 298 \2° 35′

	0.175	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
	0.106

 1.6

22.00 

0.090 

0.066 

0.1625 

  857 \2° 11′

London-	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
Bristol

 122

308 

$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	20
	20	phantom

88.00 

0.065 

0.155 

0.033  

1 550 \3° 31′

	0.175	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
	0.106

 1.125

44.00 

0.090 

0.094 

0.0255 

1 023 \2° 58′

London-	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$
Southampton   and  Portsmouth‡

  85

254 

$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	20
	20	phantom

88.00 

44

0.065 

0.090

0.222 

0.138

0.0292 

0.0235

1 855 \2° 33′

1 240 \2°  8′

	0.250	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$
	0.155

 1.125

*100-lb. conductors used for telegraphs.†Not phantomed.‡In course of construction.

(W. No.)

United States

The more important improvements made in the United States during 1910-21 are briefly described below.

Exchange Cables.—Improvements in the design and the methods of manufacture of cables for use in local exchanges made it possible greatly to increase the number of wires of a given size in a sheath of given size. By employing wires of smaller diameter than those heretofore used the maximum number was still further increased. Cables containing either 900 wires No. 19 A.W.G. (.0359 in. diam.), 1,800 wires No. 22 A.W.G. (.0253 in. diam.), or 2,400 wires No. 24 A.W.G. (.0201 in. diam.) were extensively used in 1921. The improvements which rendered practicable these cables of maximum diam. have been employed also in cables of fewer pairs, thus enabling their diams. to be decreased and their costs reduced. Cables containing the smaller sizes of wire were used as extensively as was justified by their economic balance in relation to other portions of the plant. This resulted in the employment of considerable amounts of No. 24 A.W.G. conductor cable.

For a long time cable sheaths were made of lead alloyed with about 3% of tin, unalloyed lead not having the requisite strength; and resistance to corrosion. Extensive research, directed toward finding a cheaper but no less effective alloy, resulted in 1912 in the adoption of lead alloyed with a small amount of antimony. Readjustments in the thicknesses of sheaths and in the composition of the insulating and binding paper produced still further economies.

Loading Coils in Exchange Service.—Many thousands of trunk circuits in multi-office exchanges and circuits connecting large cities with suburban points have been equipped with loading coils, pro-