[Illustration:  Fig. 209.  Cook Air-Gap Arrester]

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.  The impedance coils are wound on insulating rods, which hold them near, but not touching, the ground carbon.  The coils are arranged so that they may be turned when discharges roughen the surfaces of the wires.

[Illustration:  Fig. 214.  Holtzer-Cabot Arrester]

Metallic Electrodes:—­Copper or other metal blocks with roughened surfaces separated by an insulating slip may be substituted for the carbon blocks of most of the arresters previously described.  Metal blocks lack the advantage of carbon in that the latter allows discharges at lower potentials for a given separation, but they have the advantage that a conducting dust is not thrown off from them.