connection close to plates connected with the
line and is adapted to protect apparatus either connected across a
metallic circuit or in series with a single wire circuit.

Fig. 201 shows another form of metal plate air-gap arrester having the
further possibility of a discharge taking place from one line wire to
the other. Inserting a plug in the hole between the two line plates
connects the line wires directly together at the arrester. This
practice was designed for use with series lines, the plug
short-circuiting the telephone set when in place.

A defect of most ordinary types of metal air-gap lightning arresters
is that heavy discharges tend to melt the teeth or edges of the
plates and often to weld them together, requiring special attention
to re-establish the necessary gap.

Advantages of Carbon:--Solid carbon is found to be a much better
material than metal for the reasons that a discharge will not melt it
and that its surface is composed of multitudes of points from which
discharges take place more readily than from metals.

[Illustration Fig. 204. Saw-Tooth Arrester]

[Illustration Fig. 205. Carbon Block Arrester]

Carbon arresters now are widely used in the general form shown in Fig.
205. A carbon block connected with a wire of the line is separated
from a carbon block connected to ground by some form of insulating
separator. Mica is widely used as such a separator, and holes of some
form in a mica slip enable the discharge to strike freely from block
to block, while preventing the blocks from touching each other.
Celluloid with many holes is used as a separator between carbon
blocks. Silk and various special compositions also have their uses.

[Illustration Fig. 206. Arrester Separators]

Dust Between Carbons:--Discharges between the carbon blocks tend to
throw off particles of carbon from them. The separation between the
blocks being small--from .005 to .015 inch--the carbon particles may
lodge in the air-gap, on the edges of the separator, or otherwise, so
as to leave a conducting path between the two blocks. Slight moisture
on the separator may help to collect this dust, thus placing a ground
on that wire of the line. This ground may be of very high resistance,
but is probably one of many such--one at each arrester connected to
the line. In special forms of carbon arresters an attempt has been
made to limit this danger of grounding by the deposit of carbon dust.
The object of the U-shaped separator