The electrostatic capacity between the conductors of a telephone line
depends upon their surface area, their length, their position, and the
nature of the materials separating them from each other and from other
things. For instance, in an open wire line of two wires, the
electrostatic capacity depends upon the diameter of the wires, upon
the length of the line, upon their distance apart, upon their distance
above the earth, and upon the specific inductive capacity of the air.
Air being so common an insulating medium, it is taken as a convenient
material whose specific inductive capacity may be used as a basis of
reference. Therefore, the specific inductive capacity of air is taken
as unity. All solid matter has higher specific inductive capacity than
air.

The electrostatic capacity of two open wires .165 inch diameter, 1
ft. apart, and 30 ft. above the earth, is of the order of .009
microfarads per mile. This quantity would be higher if the wires were
closer together; or nearer the earth; or if they were surrounded by a
gas other than the air or hydrogen; or if the wires were insulated not
by a gas but by any solid covering. As another example, a line
composed of two wires of a diameter of .036 inch, if wrapped with
paper and twisted into a pair as a part of a telephone-cable, has a
mutual electrostatic capacity of approximately .08 microfarads per
mile, this quantity being greater if the cable be more tightly
compressed.

The use of paper as an insulator for wires in telephone cables is due
to its low specific inductive capacity. This is because the insulation
of the wires is so largely dry air. Rubber and similar insulating
materials give capacities as great as twice that of dry paper.