ster]

A form of Western Electric arrester particularly adapted for outside
use on railway lines is shown with its cover in Fig. 210.

[Illustration: Fig. 210. Western Electric Air-Gap Arrester]

The Kellogg Company regularly equips its magneto telephones with
air-gap arresters of the type shown in Fig. 211. The two line plates
are semicircular and of metal. The ground plate is of carbon,
circular in form, covering both line plates with a mica separator.
This is mounted on the back board of the telephone and permanently
wired to the line and ground binding posts.

[Illustration: OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN,
SAN FRANCISCO, CALIFORNIA]

[Illustration: Fig. 211. Kellogg Air-Gap Arrester]

Vacuum Arresters:--All of the carbon arresters so far mentioned depend
on the discharge taking place through air. A given pressure will
discharge further in a fairly good vacuum than in air. The National
Electric Specialty Company mounts three conductors in a vacuum of the
incandescent lamp type, Fig. 212. A greater separation and less
likelihood of short-circuiting can be provided in this way. Either
carbon or metal plates are adapted for use in such vacuum devices. The
plates may be further apart for a given discharge pressure if the
surfaces are of carbon.

[Illustration: Fig. 212. Vacuum Arrester]

Introduction of Impedance:--It has been noted that the existence of
impedance tends to choke back the passage of lightning discharge
through a coil. Fig. 213 suggests the relation between such an
impedance and air-gap arrester. If the coil shown therein be considered
an arrangement of conductors having inductance, it will be seen that a
favorable place for an air-gap arrester is between that impedance and
the line. This fact is made known in practice by frequent damage to
aerial cables by electricity brought into them over long open wires,
the discharge taking place at the first turn or bend in the aerial
cable; this discharge often damages both core and sheath. It is well to
have such bends as near the end of the cable as possible, and turns or
goosenecks at entrances to terminals have that advantage.

[Illustration: Fig. 213. Impedance and Air-Gap]

This same principle is utilized in some forms of arresters, such as
the one shown in Fig. 214, which provides an impedance of its own
directly in the arrester element. In this device an insulating base
carries a grounded carbon rod and two impedance coils. Th