ess than .08 inch, if of hard-drawn copper,
and .064 inch, if of iron or steel. The honor of developing ways of
drawing copper wire with sufficient tensile strength for open-air uses
belongs to Mr. Thomas B. Doolittle of Massachusetts.

Lines whose lengths are limited to a few miles do not require a
conductivity as great as that of copper wire of .08-inch diameter. A
wire of that size weighs approximately 100 pounds per mile. Less than
100 pounds of copper per mile of wire will not give strength enough
for use on poles; but as little as 10 pounds per mile of wire gives
the necessary conductivity for the lines of the thousands of telephone
stations in towns and cities.

Open wires, being exposed to the elements, suffer damage from storms;
their insulation is injured by contact with trees; they may make
contact with electric power circuits, perhaps injuring apparatus,
themselves, and persons; they endanger life and property by the
possibility of falling; they and their cross-arm supports are less
sightly than a more compact arrangement.

Grouping small wires of telephone lines into cables has, therefore,
the advantage of allowing less copper to be used, of reducing the
space required, of improving appearance, and of increasing safety. On
the other hand, this same grouping introduces negative advantages as
well as the foregoing positive ones. It is not possible to talk as far
or as well over a line in an ordinary cable as over a line of two open
wires. Long-distance telephone circuits, therefore, have not yet been
placed in cables for lengths greater than 200 or 300 miles, and
special treatment of cable circuits is required to talk through them
for even 100 miles. One may talk 2,000 miles over open wires. The
reasons for the superiority of the open wires have to do with position
rather than material. Obviously it is possible to insulate and bury
any wire which can be carried in the air. The differences in the
properties of lines whose wires are differently situated with
reference to each other and surrounding things are interesting and
important.

A telephone line composed of two conductors always possesses four
principal properties in some amount: (1) conductivity of the
conductors; (2) electrostatic capacity between the conductors; (3)
inductance of the circuit; (4) insulation of each conductor from other
things.

Conductivity of Conductors. The conductivity of a wire depends upon
its material, its cross-section,