es are heard in the
telephones upon it.

[Fig. 232. Drainage Coils]

A telephone line which for a long distance is near a high-tension
transmission line may have electrostatic or electromagnetic
potentials, or both, induced upon it. If the line be balanced in its
properties, including balance by transposition of its wires, the
electrostatic induction may neutralize itself. The electromagnetic
induction still may disturb it.

_Drainage Coils_. The device shown in Fig. 232, which amounts merely
to an inductive leak to earth, is intended to cure both the snowstorm
and electromagnetic induction difficulties. It is required that its
impedance be high enough to keep voice-current losses low, while being
low enough to drain the line effectively of the disturbing charges.
Such devices are termed "drainage coils."

Electrolysis. The means of protection against the danger due to
chemical action, set forth in the preceding chapter, form such a
distinct phase of the subject of guarding property against electrical
hazards as to warrant treatment in a separate chapter devoted to the
subject of electrolysis.

[Illustration: MAIN EXCHANGE, CLEVELAND, OHIO.
Largest Four-Party Selective Ringing Switchboard in the World. Kellogg
Switchboard and Supply Co.]

CHAPTER XX

GENERAL FEATURES OF THE TELEPHONE EXCHANGE

Up to this point only those classes of telephone service which could be
given between two or more stations on a single line have been
considered. Very soon after the practical conception of the telephone,
came the conception of the telephone exchange; that is, the conception
of centering a number of lines at a common point and there terminating
them in apparatus to facilitate their interconnection, so that any
subscriber on any line could talk with any subscriber on any other
line.

The complete equipment of lines, telephone instruments, and switching
facilities by which the telephone stations of the community are given
telephone service is called a telephone exchange.

The building where a group of telephone lines center for
interconnection is called a central office, and its telephonic
equipment the central-office equipment. The terms telephone office and
telephone exchange are frequently confused. Although a telephone office
building may be properly referred to as a telephone exchange building,
it is hardly proper to refer to the telephone office as a telephone
exchange, as is frequently done. In modern p