As the conductivity or resistance of a line is the property of controlling importance in telegraphy, a similar relation was expected in early telephony.  As the current in the telephone line varies rapidly, certain other properties of the line assume an importance they do not have in telegraphy in any such degree.

The importance that these properties assume is, that if they did not act and the resistance of the conductors alone limited speech, transmission would be possible direct from Europe to America over a pair of wires weighing 200 pounds per mile of wire, which is less than half the weight of the wire of the best long-distance land lines now in service.  The distance from Europe to America is about twice as great as the present commercial radius by land lines of 435-pound wire.  In other words, good speech is possible through a mere resistance twenty times greater than the resistance of the longest actual open-wire line it is possible to talk through.  The talking ratio between a mere resistance and the resistance of a regular telephone cable is still greater.

Electrostatic Capacity.  It is the possession of electrostatic capacity which enables the condenser, of which the Leyden jar is a good example, to be useful in a telephone line.  The simplest form of a condenser is illustrated in Fig. 28, in which two conducting surfaces are separated by an insulating material.  The larger the surfaces, the closer they are together; and the higher the specific inductive capacity of the insulator, the greater the capacity of the device.  An insulator used in this relation to two conducting surfaces is called the dielectric.

[Illustration:  Fig. 28.  Simple Condenser]

[Illustration:  Fig. 29.  Condenser Symbols]

Two conventional signs are used to illustrate condensers, the upper one of Fig. 29 growing out of the original condenser of two metal plates, the lower one suggesting the thought of interleaved conductors of tin foil, as for many years was the practice in condenser construction.

With relation to this property, a telephone line is just as truly a condenser as is any other arrangement of conductors and insulators.  Assume such a line to be open at the distant end and its wires to be well insulated from each other and the earth.  Telegraphy through such a line by ordinary means would be impossible.  All that the battery or other source could do would be to cause current to flow into the line for an infinitesimal time, raising the wires to its potential, after which no current would flow.  But, by virtue of electrostatic capacity, the condition is much as shown in Fig. 30.  The condensers which that figure shows bridged across the line from wire to wire are intended merely to fix in the mind that there is a path for the transfer of electrical energy from wire to wire.

[Illustration:  Fig. 30.  Line with Shunt Capacity]