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= /1/2 /(R^{2}+L^{2}[omega]^{2})(S^{