Professor Pupin likens the transmission of electric waves over long-distance circuits to the transmission 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 pass 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.

[Illustration:  MAIN ENTRANCE AND PUBLIC OFFICE, SAN FRANCISCO HOME TELEPHONE COMPANY Contract Department on Left.  Accounting Department on Right.]

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.

[Illustration:  Fig. 35.  Loaded Line]

The loading of open wires is not as successful 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 transmission; 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 passing over the line.  The equation is

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a=  12  (R^{2}+L^{2}[omega]^{2})(S^{2}+C^{2}[omega]^{2} + 12(RS-LC[omega]^{2}
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The quantities are