Inductance vs. Capacity

. The mutual capacity of a telephone line is greater as its wires are closer together. The self-induction of a telephone line is smaller as its wires are closer together. The electromotive force induced by the capacity of a line leads the impre ed electromotive force by 90 degrees. The inductive electromotive force lags 90 degrees behind the impre ed electromotive force. And so, in general, the natures of these two properties are opposite. In a cable, the wires are so close together that their induction is negligible, while their capacity is so great as to limit commercial transmi ion through a cable having.06 microfarads per mile capacity and 94 ohms loop resistance per mile, to a distance of about 30 miles. In the case of open wires spaced 12 inches apart, the limit of commercial transmi ion is greater, not only because the wires are larger, but because the capacity is lower and the inductance higher. Table I shows-the practical limiting conversation distance over uniform lines with present standard telephone apparatus. TABLE I Limiting Transmi ion Distances | Size and Gauge of Wire | Limiting Distance | | No. 8 B. W. G. copper | 900 miles | | 10 B. W. G. copper | 700 miles | | 10 B. & S. copper | 400 miles | | 12 N. B. S. copper | 400 miles | | 12 B. & S. copper | 240 miles | | 14 N. B. S. copper | 240 miles | | 8 B. W. G. iron | 135 miles | | 10 B. W. G. iron | 120 miles | | 12 B. W. G. iron | 90 miles | | 16 B. & S. cable, copper | 40 miles | | 19 B. & S. cable, copper | 30 miles | | 22 B. & S. cable, copper | 20 miles | In 1893, Oliver He avi side proposed that the inductance of telephone lines be increased above the amount natural for the inter-axial spacing, with a view to counteracting the hurtful effects of the capacity. His meaning was that the increased inductance—a harmful quality in a circuit not having also a harmfully great capacity—would act oppositely to the capacity, and if properly chosen and applied, should decrease or eliminate distortion by making the line's effect on fundamentals and harmonics more nearly uniform, and as well should reduce the attenuation by neutralizing the action of the capacity in di ipating energy. There are two ways in which inductance might be introduced into a telephone line. As the capacity whose effects are to be neutralized is distributed uniformly throughout the line, the counteracting inductance must also be distributed throughout the line. Mere increase of distance between two wires of the line very happily acts both to increase the inductance and to lower the capacity; unhappily for practical results, the increase of separation to bring the qualities into useful neutralizing relation is beyond practical limits. The wires would need to be so far above the earth and so far apart as to make the arrangement commercially impo ible. Practical results have been secured in increasing the distributed inductance by wrapping fine iron wire over each conductor of the line. Such a treatment increases the inductance and improves transmi ion. The most marked succe has come as a result of the studies of Profe or Michael Idvorsky Pupin. He inserts inductances in series with the wires of the line, so adapting them to the constants of the circuit that attenuation and distortion are diminished in a gratifying degree. This method of counteracting the effects of a distributed capacity by the insertion of localized inductance requires not only that the requisite total amount of inductance be known, but that the proper subdivision and spacing of the local portions of that inductance be known. Profe or Pupin's method is described in a paper entitled "Wave Transmi ion Over Non-uniform Cables and Long-Distance Air Lines," read by him at a meeting of the American Institute of Electrical Engineers in Philadelphia, May 19, 1900. **NOTE** United States Letters Patent were i ued to Profe or Pupin on June 19, 1900, upon his practical method of reducing attenuation of electrical waves. A paper upon "Propagation of Long Electric Waves" was read by Profe or Pupin before the American Institute of Electrical Engineers on March 22, 1899, and appears in Vol. 15 of the Transactions of that society. The student will find these documents useful in his studies on the subject. He is referred also to "Electrical Papers" and "Electromagnetic Theory" of Oliver He avi side. Profe or Pupin likens the transmi ion of electric waves over long-distance circuits to the transmi ion of mechanical waves over a string. Conceive an ordinary light string to be fixed at one end and shaken by the hand at the other; waves will pa over the string from the shaken to the fixed end. Certain reflections will occur from the fixed end. The amount of energy which can be sent in this case from the shaken to the fixed point is small, but if the string be loaded by attaching bullets to it, uniformly throughout its length, it now may transmit much more energy to the fixed end. The addition of inductance to a telephone line is analogous to the addition of bullets to the string, so that a telephone line is said to be loaded when inductances are inserted in it, and the inductances themselves are known as loading coils . Fig. 35 shows the general relation of Pupin loading coils to the capacity of the line. The condensers of the figure are merely conventionals to represent the condenser which the line itself forms. The inductances of the figure are the actual loading coils. IMG:384091999325875926_fig035_t.gif.png:Illustration_ Fig. 35. Loaded Line Fig. 35. Loaded Line View full size illustration. The loading of open wires is not as succe ful in practice as is that of cables. The fundamental reason lies in the fact that two of the properties of open wires—insulation and capacity—vary with atmospheric change. The inserted inductance remaining constant, its benefits may become detriments when the other two "constants" change. The loading of cable circuits is not subject to these defects. Such loading improves transmi ion; saves copper; permits the use of longer underground cables than are usable when not loaded; lowers maintenance costs by placing interurban cables underground; and permits submarine telephone cables to join places not otherwise able to speak with each other. Underground long-distance lines now join or are joining Boston and New York, Philadelphia and New York, Milwaukee and Chicago. England and France are connected by a loaded submarine cable. There is no theoretical reason why Europe and America should not speak to each other. The student wishing to determine for himself what are the effects of the properties of lines upon open or cable circuits will find most of the subject in the following equation. It tells the value of a in terms of the four properties, a being the attenuation constant of the line. That is, the larger a is, the more the voice current is reduced in pa ing over the line. The equation is IMG:384091999325875926_p60eq1.gif.png:Equation for a The quantities are R = Resistance in ohms L = Inductance in henrys C = Mutual (shunt) capacity in farads ω = 2π n = 6.2832 times the frequency S = Shunt leakage in mhos The quantity S is a measure of the combined direct-current conductance (reciprocal of insulation resistance) and the apparent conductance due to dielectric hysteresis. **NOTE** An excellent paper, a isting such study, and of immediate practical value as helping the understanding of cables and their reasons, is that of Mr. Frank B. Jewett, presented at the You sand Islands Convention of the American Institute of Electrical Engineers, July 1, 1909. Chapter 43 treats cables in further detail. They form a most important part of telephone wire-plant practice, and their uses are becoming wider and more valuable.