If a specific case of such exposure indicates that the cables may be in danger, the long open lines then are equipped with additional air-gap arresters at the point of junction of those open lines with the cable.  Practice varies as to the type.  Maintenance charges are increased if carbon arresters separated .005 inch are used, because of the cost of sending to the end of the long cable to clear the blocks from carbon dust after each slight discharge.  Roughened metal blocks do not become grounded as readily as do carbon blocks.  The occasions of visit to the arresters, therefore, usually follow actual heavy discharges through them.

The recommendations and the practice of the American Telephone and Telegraph Company differ on this point, while the practice of other companies varies with the temperaments of the engineers.  The American Company specifies copper-block arresters where long country lines enter cables, if those lines are exposed to lightning discharges only.  The exposed line is called long if more than one-half mile in length.  If it is exposed to high-potential hazards, carbon blocks are specified instead of copper.  Other specifications of that company have called for the use of copper-block arresters on lines exposed to hazards above 2,500 volts.

[Illustration:  ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED MULTIPLE BOARD OF THE CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO Ultimate Capacity, 24,000 Lines.  One of the Two Examples in the United States of a Multiple Switchboard Having an Ultimate Capacity over 18,000 Lines.  Replaced Recently by a Kellogg Straight Multiple Board Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of 10,000 Lines.]

The freedom of metal-block arresters from dust troubles gives them a large economical advantage over carbon.  For similar separations, the ratio of striking voltages between carbon blocks and metal blocks respectively is as 7 to 16.  In certain regions of the Pacific Coast where the lightning hazard is negligible and the high tension hazard is great, metal-block arresters at the outer ends of cables give acceptable protection.

High winds which drive snow or dust against bare wires of a long line, create upon or place upon those wires a charge of static electricity which makes its way from the line in such ways as it can.  Usually it discharges across arresters and when this discharge takes place, the line is disturbed in its balance and loud noises are heard in the telephones upon it.

[Fig. 232.  Drainage Coils]

A telephone line which for a long distance is near a high-tension transmission line may have electrostatic or electromagnetic potentials, or both, induced upon it.  If the line be balanced in its properties, including balance by transposition of its wires, the electrostatic induction may neutralize itself.  The electromagnetic induction still may disturb it.

Drainage Coils.  The device shown in Fig. 232, which amounts merely to an inductive leak to earth, is intended to cure both the snowstorm and electromagnetic induction difficulties.  It is required that its impedance be high enough to keep voice-current losses low, while being low enough to drain the line effectively of the disturbing charges.  Such devices are termed “drainage coils.”