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