ELECTRICAL AND OPTICAL PHENOMENA.

 
   

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of the same kind, repel one another; if charged with dif-
ferent kinds, they attract, and if the bodies are free to
move, they approach, when the opposite excitements neu-
tralize each other. In case the electrical excitement is
considerable, the union is accompanied by a sharp crack,
and a flash of light, called the electric spark.

291. Conductors of electricity are bodies which
allow its ready passage through them. Metals,
charcoal, acids, aqueous solutions, and various
animal and vegetable substances, are good con-
ductors. Non-conductors are those which do not
allow the electricity to flow freely through them.
Gums, resins, glass, silk, and dry air are non-con-
ductors.

The higher the conducting power of a circuit the lower
will be its resistance, and, consequently, the greater the
current which will be sent through it by a given poten-
tial. ; :

292. Atmospheric Electricity —Electric excite-
ment is always present in the atmosphere. The
electricity of the air is generally positive, although
it often changes rapidly to negative on the ap-
proach of clouds or fogs. It is feeblest within a
few feet of the surface, and increases with the
elevation above the general surface of the earth.

Origin of Free Atmospheric Hlectricity.—The elec-
tricity of the atmosphere is caused by a variety of circum-
stances, the chief of which are evaporation and condensa-
tion; unequal heating of the earth by the sun’s rays;
combustion; animal and vegetable life; and the friction

of winds against each other or against the earth’s sur-
face.

293. Lightning occurs when the electricity of
a cloud discharges to the earth or to a neighbor-
ing cloud. The discharge is attended by a vivid
spark, called lightning. The destructive effects
of lightning are due to the discharge between the
clouds and the earth.

Thunder.—The heat of the spark vaporizes the
rain-drops, and enormously expands the air, pro-
ducing, on their subsequent cooling, a partial
vacuum, which is further increased by the mo-
‘mentary pushing aside of the air by the discharge.
The surrounding air rushing violently into this
vacuum produces the sound called thunder.

The potential of the lightning flash is enormously higher
than that produced by artificial means, and must be equal
to many millions of volts. This high potential is due to
the enormous decrease in the surface of a single rain-drop
from the thousands of smaller drops which have coalesced.
to form it.

294, Varieties of Lightning.—There are five varieties
of lightning: zig-zag or chain, sheet, heat, globular, and vol-
canic lightning.

Zig-zag Lightning probably owes its forked shape to
the resistance which the air offers to its passage through

 

it. The air-particles, being crowded together in the path
of the spark, the lightning darts to one side, where the air
is less dense.

Sheet Lightning generally accompanies thunder-
storms, and appears as an expanded flash, which illu-
mines the clouds.

Heat Lightning, or lightning without thunder, is gener-
ally seen near the horizon, during hot weather. It is
probably caused by the reflection of lightning from a
storm below the horizon.

Globular Lightning. On rare occasions, the lightning
appears in the form of a globe of light, which remains
stationary in the air or moves slowly through it. Its
cause is unknown.

Volcanic Lightning. During the eruption of volca-
noes, vivid flashes of lightning often occur in the air near
the craters. Volcanic lightning is probably caused by the
rapid condensation of the vast volumes of vapor emitted
with the ashes and lava.

295. Lightning Rods, invented by Franklin,
protect the buildings on which they are placed, by
quietly discharging the electricity from the over-
hanging cloud. They generally effect this by an
opposite electricity passing from the earth up the
rod, and neutralizing that of the cloud. Unless
the rods are placed in good metallic connection
with the earth, and with all conductors near
them, they are sources of danger rather than of
protection.

296. St. Elmo’s Fire—When the. atmosphere
is highly charged with electricity, faint tongues
of fire are often seen on the ends of bodies in

   

Fig. 99, St Elmo's Fire,

connection with the earth, like the masts of ships,
steeples, etc., due to an electric discharge, known
as the brush-discharge. They are called St. Elmo’s
fire, and are harmless.