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, 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 resistance.  Electromotive force being a cause, and resistance a state, current is the result.  The formula of this relation, Ohm’s law, is

C = E/R

C being the current which results from E, the electromotive force, acting upon R, the resistance.  The units are:  of current, the ampere; of electromotive force, the volt; of resistance, the ohm.