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.