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, its length, and its temperature.
Conductivity of a copper wire, for example, increases in direct ratio
to its weight, in inverse ratio to its length, and its conductivity
falls as the temperature rises. Resistance is the reciprocal of
conductivity and the properties, conductivity and resistance, are more
often expressed in terms of resistance. The unit of the latter is the
_ohm_; of the former the _mho_. A conductor having a resistance of 100
ohms has a conductivity of .01 mho. The exact correlative terms are
_resistance_ and _conductance_, _resistivity_ and _conductivity_. The
use of the terms as in the foregoing is in accordance with colloquial
practice.

Current in a circuit having resistance only, varies inversely as the