A simple test will enable two of the results of a short-circuiting capacity to be appreciated.  Conceive a very short line of two wires to connect two local battery telephones.  Such a line possesses negligible resistance, inductance, and shunt capacity.  Its insulation is practically infinite.  Let condensers be bridged across the line, one by one, while conversation goes on.  The listening observer will notice that the sounds reaching his ear steadily grow less loud as the capacity across the line increases.  The speaking observer will notice that the sounds he hears through the receiver in series with the line steadily grow louder as the capacity across the line increases.  Fig. 31 illustrates the test.

The speaker’s observation in this test shows that increasing the capacity across the line increased the amount of current entering it.  The hearer’s observation in this test shows that increasing the capacity across the line decreased the amount of energy turned into sound at his receiver.

[Illustration:  Fig. 31.  Test of Line with Varying Shunt Capacity]

The unit of electrostatic capacity is the farad.  As this unit is inconveniently large, for practical applications the unit microfarad—­millionth of a farad—­is employed.  If quantities are known in microfarads and are to be used in calculations in which the values of the capacity require to be farads, care should be taken to introduce the proper corrective factor.

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.