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

500 ELECTRIC CURRENTS.
"To produce the effects, whatever these may be, by which
the telegraphic messages are expressed, it is necessary that
the electric current shall have a certain intensity. Now, the
intensity of the current transmitted by a given voltaic battery
along a given line of wire will decrease, other things being the
same, in the same proportion as the length of the wire increases.
Thus, if the wire be continued for ten miles, the current will
have twice the intensity which it would have if the wire had
been extended to a distance of twenty miles.
It is evident, therefore, that the wire may be continued to
such a length that the current will no long er have sufficient
intensity to produce at the station to which the despatch is
transmitted those effects by which the language of the despatch
is signified.
The intensity of the current transmitted by a given voltaic
battery upon a wire of given length will be increased in the
same proportion as the area of the section of the wire is augmented. Thus, if the diameter of the wire be doubled, the
area of its section being increased in a four-fold proportion, the
intensity of the current transmitted along the wire will be
increased in the same ratio.
In fine, the intensity of the current may also be augmented
by increasing the number of pairs of generating plates or cylinders composing the voltaic battery.
Since it has been found. most convenient generally to use
iron as the material for the conducting wires, it is of no practical importance to take into account the influence which the
quality of the metal may produce upon the intensity of the
current. It may be useful, nevertheless, to state that, other
things being the same, the intensity of the current will be in
proportion to the conducting power of the metal of which the
wire is formed, and that copper is the best conductor of the
metals.
3M. Pouillet found, by well-conducted experiments, that the
current supplied by a voltaic battery of ten pairs of plates,
transmitted upon a copper wire having a diameter of four onethousandths of an inch, and a length of six tenths of a mile,
was sufficiently intense for all the common telegraphic purposes. Now, if we suppose that the wire, instead of being four
one-thousandths of an inch in diameter, has a diameter of a quarter of an inch, its diameter being greater in the ratio of sixtytwo and one half to one, its section will be greater in the ratio
of nearly four thousand to one, and it will, consequently, carry
a current of equal intensity over a length of wire four thousand
times greater-that is, over two thousand four hundred miles
of wire."