Man's physical universe

xanabras

STATIC ELECTRICITY 489

poles, or buildings which rise above the surface of the ground and

thus act as pointed surfaces. Eventually this charge may become so

great that the resistance of the air is overcome and a lightning flash

occurs. To protect an object against lightning, metal rods with sharp

points are used to discharge the induced charge. Such lightning rods

must be well grounded in the earth, and it is generally recommended

that they be connected with some object such as a copper boiler

buried in the deeper damp earth. Static charges which cause sheets of

Fig. 232. Three million volts of

laboratory lightning striking a car in

the Westinghouse high voltage laboratory.

The bolt can be seen jumping

over the left front wheel to reach

the ground. (Courtesy of the Westinghouse

Electric and Manufacturing

Company.)

Fig. 233. Lightning striking the

Empire State Building, New York

City. Although this building has

been struck by lightning many times,

no harm was done to the building

because its steel framework acts as a

Faraday cage. (Courtesy of the

General Electric Company.)

discharged by draping Christmas-tree tinsel over the machine.

paper to stick together while using a mimeograph machine may be

Gasoline

trucks drag chains on the ground to prevent the accumulation of

static electricity that might produce a spark and ignite the gasoline.

The United States Census reports show that nearly two thousand

people are killed or injured by lightning each year.

When one is caught out of doors in a thunderstorm, he will be safe

under a well-grounded iron bridge. In a violent storm he should sit

down or lie down in a hollow spot away from trees or fences. In woods,

seek shelter under the smaller trees. Do not go in swimming during

a lightning storm.

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