Man's physical universe

xanabras

488 MAGNETISM AND ELECTRICITY

of the conductors. If the conductors are now separated without

moving the rod, they will be found to have opposite charges.

Charges are distributed uniformly only if the surface is smooth and

flat or uniformly curved; on other surfaces the charges collect at the

more pointed portions. Electrons concentrate on sharp points and

cause the air to become ionized; that is, molecules of air become

charged and thus conduct the electrons

from the point.

Sometimes this discharge at the tips of

lightning rods, flag poles, or masts of

ships produces a faint violent glow, called

"St. Elmo's Fire," that can be seen at

night. Similar discharges along highvoltage

electrical transmission lines are

called "corona" discharges. The ions repelled

from the charged body create a

wind which is capable of blowing a candle

Fig. 231. The electric whirl.

flame out of shape. The "electric whirl,"

which is based on the principle of the

Fourth of July pinwheel, or rotary lawn-sprinkler, is also produced by

a similar repulsion of ions from a charged body.

In operating the Van de GraafT electrostatic generators, to be described

later, the operators are housed within the charged spheres

for protection against possible harm from the tremendous voltages

produced, for charges cannot remain on the inner surface of the cage.

Lightning Is a Static Phenomenon.

As previously mentioned, lightning is the discharge between a cloud

and the earth. The source of the charges is not thoroughly understood,

although it is generally believed that it is produced by the intense

friction and splitting of large raindrops brought about by the

rapidly rising currents of air so characteristic of thunderstorms. The

rising currents of air may possibly produce differences in charges due

to the fact that the lower atmosphere is positively charged relative to

the upper atmosphere. Very possibly the charge is also produced by the

rapid condensation of moisture in such storms, the charges being concentrated

on smaller total areas and thus producing higher potentials.

Negative charges concentrate on the bottoms of clouds, leaving the

tops positively charged.

Discharges may take place between the top

and bottom surfaces of adjoining clouds.

A charged cloud may induce an opposite charge on the nearest portion

of the earth. This induced charge will concentrate on trees, flag

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