Man's physical universe

xanabras

490 MAGNETISM AND ELECTRICITY

If a house could be enclosed in a Faraday cage of ironwork, the house

would not be struck by lightning, and the cage could be struck repeatedly

without producing any observable effect. Modern steelframed

skyscrapers with their foundations sunk deep into the wet

ground constitute such cages. Such structures are often struck by

lightning, but there is no record of anyone ever havirtg been harmed

in the process. Similarly, a cage of cables is very effective for smaller

dwellings as a protection against lightning.

The passage of lightning through the air raises the temperature of

the air and thus causes such a great expansion of gas in a small amount

of time that compressional w'aves are produced similar to those produced

by a tremendous explosion, thus producing thunder.

Machines Have Been Invented to Produce Static Electricity.

Fig. 234. Artificial lightning pro

duced by the Van de Graaff gener

ator. (Courtesy of the Massachu

setts Institute of Technology'.)

Static electricity may be produced by means of various electrostatic

machines, such as the Wimshurst machine, which consists of many

strips of tin foil mounted on a nonconducting

disk. These disks are

rotated and produce charges by induction.

The charges are collected

by metallic brushes and carried to a

Leyden jar, where they are stored.

The principle of the Leyden jar will

be discussed in the following description

of condensers.

The production of static electricity

by induction has been carried out

on a large scale by Van de Graaff

of the Massachusetts Institute of

Technology.

The 10,000,000-volt generator constructed

by Van de GraafT consists

of two large hollow columns, 25 feet

high and 6 feet in diameter, surrounded

at the top by hollow polished

aluminum spheres 15 feet in diameter.

Each column is supported on a

heavy four-wheeled truck running on a railroad track 14 feet wide.

Inside of each ball is a pulley, and a corresponding pulley is mounted

on the shaft of a motor at the bottom of the column. A silk belt

travels around these pulleys. A 10,000-volt transformer-rectifier set

is used to deliver a 10,000-volt direct current to the belts by means

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