[Illustration:  Fig. 26.  Lamp Signal Directly in Line]

[Illustration:  Fig. 27.  Lamp Signal and Ballast]

The nature of carbon and certain earths being that their conductivity rises with the temperature and that of metals being that their conductivity falls with the temperature, has enabled the Nernst lamp to be successful.  The same relation of properties has enabled incandescent-lamp signals to be connected direct to lines without relays, but compensated against too great a current by causing the resistance in series with the lamp to be increased inversely as the resistance of the filament.  Employment of a “ballast” resistance in this way is referred to in Chapter XI.  In Fig. 27 is shown its relation to a signal lamp directly in the line. 1 is the carbon-filament lamp; 2 is the ballast.  The latter’s conductor is fine iron wire in a vacuum.  The resistance of the lamp falls as that of the ballast rises.  Within certain limits, these changes balance each other, widening the range of allowable change in the total resistance of the line.

CHAPTER IV

TELEPHONE LINES

The line is a path over which the telephone current passes from telephone to telephone. The term “telephone line circuit” is equivalent.  “Line” and “line circuit” mean slightly different things to some persons, “line” meaning the out-of-doors portion of the line and “line circuit” meaning the indoor portion, composed of apparatus and associated wiring.  Such shades of meaning are inevitable and serve useful purposes.  The opening definition hereof is accurate.

A telephone line consists of two conductors.  One of these conductors may be the earth; the other always is some conducting material other than the earth—­almost universally it is of metal and in the form of a wire.  A line using one wire and the earth as its pair of conductors has several defects, to be discussed later herein.  Both conductors of a line may be wires, the earth serving as no part of the circuit, and this is the best practice.  A line composed of one wire and the earth is called a grounded line; a line composed of two wires not needing the earth as a conductor is called a metallic circuit.

In the earliest telephone practice, all lines were grounded ones.  The wires were of iron, supported by poles and insulated from them by glass, earthenware, or rubber insulators.  For certain uses, such lines still represent good practice.  For telegraph service, they represent the present standard practice.

Copper is a better conductor than iron, does not rust, and when drawn into wire in such a way as to have a sufficient tensile strength to support itself is the best available conductor for telephone lines.  Only one metal surpasses it in any quality for the purpose:  silver is a better conductor by 1 or 2 per cent.  Copper is better than silver in strength and price.