If a specific case of such exposure indicates that the cables may be
in danger, the long open lines then are equipped with additional
air-gap arresters at the point of junction of those open lines with
the cable. Practice varies as to the type. Maintenance charges are
increased if carbon arresters separated .005 inch are used, because of
the cost of sending to the end of the long cable to clear the blocks
from carbon dust after each slight discharge. Roughened metal blocks
do not become grounded as readily as do carbon blocks. The occasions
of visit to the arresters, therefore, usually follow actual heavy
discharges through them.

The recommendations and the practice of the American Telephone and
Telegraph Company differ on this point, while the practice of other
companies varies with the temperaments of the engineers. The American
Company specifies copper-block arresters where long country lines
enter cables, if those lines are exposed to lightning discharges only.
The exposed line is called _long_ if more than one-half mile in
length. If it is exposed to high-potential hazards, carbon blocks are
specified instead of copper. Other specifications of that company have
called for the use of copper-block arresters on lines exposed to
hazards above 2,500 volts.

[Illustration: ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED
MULTIPLE BOARD OF THE CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO
Ultimate Capacity, 24,000 Lines. One of the Two Examples in the United
States of a Multiple Switchboard Having an Ultimate Capacity over
18,000 Lines. Replaced Recently by a Kellogg Straight Multiple Board
Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of
10,000 Lines.]

The freedom of metal-block arresters from dust troubles gives them a
large economical advantage over carbon. For similar separations, the
ratio of striking voltages between carbon blocks and metal blocks
respectively is as 7 to 16. In certain regions of the Pacific Coast
where the lightning hazard is negligible and the high tension hazard
is great, metal-block arresters at the outer ends of cables give
acceptable protection.

High winds which drive snow or dust against bare wires of a long line,
create upon or place upon those wires a charge of static electricity
which makes its way from the line in such ways as it can. Usually it
discharges across arresters and when this discharge takes place, the
line is disturbed in its balance and loud nois