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wires, cable, line, cables, telephone, miles, time, underground and lead

DATE • No. 22 gauge wires 1900 300 and 400 pairs of wires 1903 600 pairs of wires 1912 900 pairs of wires During 1914, there was developed a type of un derground cable carrying 1,200 pairs (2,400 wires) No. 24 B. & S. G. in size (0.0201) inches in diameter). Fig. 48 shows a cross-section (reduced in size) of this cable. The improve ment which it represents may be understood when it is known that to carry the same num ber of open wires on poles would require 8 huge pole lines of the size. shown in Fig. 45. The economies accomplished by these improve ments are not limited to the cables themselves, but extend to the underground duct systems whose capacity is multiplied enormously by the increase in the number of wires which each cable may carry.

It was early found that lead alone did not possess the requisite corrosion-resisting and strength properties for underground cable sheaths. Aerial cables, furthermore, are subject to peculiar conditions causing vibration in the cable, and a tendency to crystallization of the sheath if pure lead is employed. For a long period, cable sheaths having the proper qualities were made of lead alloyed with about 3 per cent of tin. Considerable increases in the cost of tin led to experimental investigations of a wide range of other alloys. This resulted in finding, in 1912, a cheaper and at least equally efficient alloy in which the lead was alloyed with a very small amount of antimony.

Early Interurban. In the earliest interurban cables every effort of the engineers was bent toward improving transmission effi ciency by using wires of large size so arranged in cabling as to secure a soft core giving the lowest practicable mutual electrostatic capacity between the wires of a pair. Wires as large as 0.095 inches in diameter were used and cables of this character as long as 25 miles were placed. In European practice, the use of wires even larger than this continued for many years. In the United States, the development was in the direction of using smaller wires provided with means for improving their transmission efficiency so that it not only equaled, but con siderably exceeded that obtainable with the larger and more expensive wires. By 1902, the art had so advanced, by the use of Pupin load ing coils (described below) and other improve ments, that a loaded cable for suburban toll service was successfully installed between New York and Newark. By 1905, a loaded cable 20 miles long had been extended from New York in the direction of Philadelphia, and by 1906, a cable 90 miles long was successfully operated between those two cities, but in the then state of the art, that cable could not be used beyond Philadelphia or New York Underground Cable. — During the year 1913 such advances were made in the art of loading and balancing under ground circuits, and such improvements in the use of repeaters (described below) that it be came possible to talk satisfactorily by under ground wires from Boston to Washington, a distance of about 450 miles, employing a cable (Fig. 49) so designed that the phantom prin

ciple could be employed in it. At the pres ent time, there are underground cables along this whole route, furnishing service of the highest reliability between many important cities, the number of cables varying from two to four, depending on the density of the telephone traffic. Loading is used on all the circuits in these cables, and repeaters are applied to a large proportion of the circuits over 60 miles in length. The Boston-Washington telephone cable is several times longer than any other in the world. These developments tended to increase greatly the long-distance traffic and to ac complish enormous savings in the amounts of copper which otherwise would have been re quired to establish communication between re mote points.

Telephone Transmission teleph ony, as already described, mechanical vibra tions of the transmitter diaphragm set up elec trical oscillations. These are transmitted over the circuit and set up corresponding mechanical vibrations of the receiver diaphragm. The fol lowing section deals with the electrical charac teristics of the telephone circuit and the trans mission of telephone currents over it.

To develop the principles of telephone trans mission, it is necessary to know the character of the speech waves. For ordinary practical working, satisfactory results are obtained on the basis that speech waves, largely the over tones of the voice, consist of a varying mixture of sin,gle frequency tones ranging from abont 400 cycles per second (in musical sound, equiva lent to G above middle C) to about 2,030 cycles per second (approximately the octave above high C), and that each frequency persists in a constant state for a sufficient length of time to follow the same electrical laws as though it persisted for a long time. This basis is by no means exact, since frequencies outside the above range are of some importance in speeth, and speech is so irregular and discontinuous in char acter that the transient oscillations, which are produced by the rapidly changing speech waves, are in many cases of considerable importance.

A telephone line possesses four °linear con stants)) on which depend the character of trans mission over it. These, with the symbols em ployed to represent them in the formulm which follow, are: Resistance in series with the line. Symbol R. Inductance in series with the line: Symbol L. Capacitance shunted across the line: Sym bol C.

Conductance shunted across the line: Sm. bol G.

The differential equation determining the trans mission of current over the line with these con stants is as follows, °P) standing for the cur rent, "xo for the distance measured along the line and ((to for the time: