of Fig. 206 is to enable the
arrester to be mounted so that this opening in the separator is
downward, in the hope that loosened carbon particles may fall out of
the space between the blocks. The deposit of carbon on the inside
edges of the U-shaped separator often is so fine and clings so tightly
as not to fall out. The separator projects beyond the blocks so as to
avoid the collection of carbon on the outer edges.

Commercial Types:--Fig. 207 is a commercial form of the arrangement
shown in Fig. 205 and is one of the many forms made by the American
Electric Fuse Company. Line wires are attached to outside binding
posts shown in the figure and the ground wire to the metal binding
post at the front. The carbon blocks with their separator slide
between clips and a ground plate. The air-gap is determined by the
thickness of the separator between the carbon blocks.

[Illustration: Fig. 207. Carbon Block Arrester]

[Illustration: Fig. 208 Roberts "Self-Cleaning" Arrester]

The Roberts carbon arrester is designed with particular reference to
the disposal of carbon dust and is termed self-cleaning for that
reason. The arrangement of carbons and dielectric in this device is
shown in Fig. 208; mica is cemented to the line carbon and is large
enough to provide a projecting margin all around. The spark gap is not
uniform over the entire surface of the block but is made wedge-shaped
by grinding away the line carbon as shown. It is claimed that a
continuous arcing fills the wedge-shaped chamber with heated air or
gas, converting the whole of the space into a field of low resistance
to ground, and that this gas in expanding drives out every particle of
carbon that may be thrown off. It seems obvious that the wedge-shaped
space offers greater freedom for carbon dust to fall out than in the
case of the parallel arrangement of the block faces.

An outdoor arrester for metallic circuits, designed by F.B. Cook, is
shown in Fig. 209. The device is adapted to mount on a pole or
elsewhere and to be covered by a protecting cap. The carbons are large
and are separated by a special compound intended to assist the
self-cleaning feature. The three carbons being grouped together as a
unit, the device has the ability to care for discharges from one
terminal to either of the others direct, without having to pass
through two gaps. In this particular, the arrangement is the same as
that of Fig. 204.

[Illustration: Fig. 209. Cook Air-Gap Arre