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

516 ELECTRIC TELEGRAPH CONDUCTORS.
very small metallic substance to serve as a conductor. These
corroded wires have frequently been replaced by new ones, and
the increased facility in telegraphing at once realized. To
remedy their rapid decay, zinc coated wires have been adopted,
and their durability is greatly extended; nevertheless, in time,
they too yield to the devouring elements; the sulphurous vapors, passing over the oxyde of zinc covering, convert it into sulphate of zinc, which-being soluble in water, is immediately
dissolved by the rain and drops off. The wire being thus deprived of its insoluble armor, rapidly corrodes.
ADVANTAGES OF ZINC-COATED WIRES.
Many of the American lines have in use zinc-coated wirescommonly but improperly called " galvanized "-and their use
has given great satisfaction. The advantages realized from
the use of the zinc-coated wires, in the perfection of the joints, are
sufficient to compensate for their general adoption. The economy to any company resulting from this one point of consideration is more than can be estimated by comparative values.
Besides this, the wire for the whole line is preserved in its full
metallic surface, and its conductibility is made even and continuous. On a line of 300 miles, if one mile of the line wire be
reduced in size from that of the other 299 miles, the one mile of
faulty wire will be a continual retardation to the flow of the
current on the 299 miles of good wire. The trials given zinccoated wire have established, beyond doubt, very great advantages in favor of its use for telegraphic purposes.
Objections have been made to the use of zinc-coated wire,
in the Southwest, especially across prairies, where there are no
trees to serve as auxiliaries in conducting the atmospheric
electricity to the earth. A. telegraph wire traversing forests
can not be disturbed by atmospheric electricity, while on the
other hand, when it traverses open fields, or prairies, it is very
liable to serious interruption from that source. The use of the
zinc coated wire, across these open plains, affords a greater
metallic surface, for the atmospheric electricity. If tire iron
wire was of equal size without the zinc, the result would be
in proportion to the conductibility of iron and zinc. It is not
the zinc that induces the. atmospheric electricity to localize
upon the line wire. The conductibility of zinc is 3-1 and that
of iron is 5_-, The zinc, it is true, has a great surface or circumference, but that additional surface does not give it an
equal power with the iron. It cannot be maintained, therefore,
that the zinc is at fault in the premises. If the wire was copper, the interference would be much greater than with the iron