Air-Gap Arrester.  The tendency of a winding to oppose lightning discharges and the ease with which such discharge may strike across insulating gaps, points the way to protection against them.  Such devices consist of two conductors separated by an air space or other insulator and are variously known as lightning arresters, spark gaps, open-space cutouts, or air-gap arresters.  The conductors between which the gap exists may be both of metal, may be one of metal and one of carbon, or both of carbon.  One combination consists of carbon and mercury, a liquid metal.  The space between the conductors may be filled with either air or solid matter, or it may be a vacuum.  Speaking generally, the conductors are separated by some insulator.  Two conductors separated by an insulator form a condenser.  The insulator of an open-space arrester often is called the dielectric.

[Illustration Fig. 203.  Saw Tooth Arrester]

Discharge Across Gaps:—­Electrical discharges across a given distance occur at lower potentials if the discharge be between points than if between smooth surfaces.  Arresters, therefore, are provided with points.  Fig. 203 shows a device known as a “saw-tooth” arrester because of its metal plates being provided with teeth.  Such an arrester brings a ground 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.