The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

VELOCITY OF THE CURRENTS. 50.5
A subsequent experiment, conducted by Professor Walker,
on some of the overground wires comprised in the American
system, gives the velocity of the voltaic current, through twohundred-and-fifty mile circuits, at about sixteen thousand miles
per second.
The underground wires, however, as just mentioned, give a
far lower result; and hence it appears evident that the velocity
of frictional electricity far exceeds the voltaic or magnetic current, owing, doubtless, to the far greater intensity and comparatively small quantitative development of the former.
The retardation experienced in underground wires, as regards the propagation of the electric impulse, is not, however,
due to any resistance of the conducting medium; for, as it is
found, in the instance of the Leyden jar, that the frictional
electricity communicated is temporarily absorbed by the metal
in the interior of the jar, so the galvanic or magnetic currents,
during their passage through the underground wires, are partly
absorbed, until the mass of copper constituting the wire is
saturated with electricity; and it would also appear that a
definite time is occupied in the absorption of the electricity by
the successive portions of the wire, such as is found to occur in
charging a Leyden jar; and until this process of impregnation
has been completed, the sensation cannot be communicated to
the other end of the conductor.
The retardation will, therefore, result, not from resistance,
but froin the first portion of the charge communicated being
absorbed for the time by the conductor through which it passes;
for, in addition to the foregoing, copper wire conducts far more
freely than the iron wire made use of in the overground wires.
Consequently the speed with which an electric impulse is
communicated varies with the energy or intensity of the current employed, and the nature or conditions of the conductor
interposed."
In relation to this subject, the following question among
others, was propounded to Mr. Charles T. Bright, the engineer
of the late Atlantic Telegraph Company, and his answer to the
same is herewith given, viz.:
" 43. What do you consider return currents? and to what
extent do you find the existence of the same on both overground
and underground lines? Please state all the points fully.
Answer 43d. On overground lines they are very trifling, indeed, compared with underground; the conditions on which the
wires are suspended and insulated, passing also through a medium, capable, to a certain extent, of absorbing any electricity
developed in surplus, prevents the occurrence of any effects
appreciable by ordinary needle telegraphic instruments.