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

-----------------------------------------------------------------------
/ -----------------------------------------------
a= /½ /(R^{2}+L^{2}[omega]^{2})(S^{2}+C^{2}[omega]^{2} + ½(RS-LC[omega]^{2}
\/ \/


The quantities are