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.

Fig. 209. Cook Air-Gap Arrester
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A form of Western Electric arrester particularly adapted for outside use on railway lines is shown with its cover in Fig. 210.

Fig. 210. Western Electric Air-Gap Arrester
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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.