Picture - Cosmic Polymath

ATMOSPHERIC AND OTHER ELECTRICITY 119
seat of the phenomena in real actions going on in the medium ; they were satisfied that they had found it in a
power of action at a distance impressed on the electric fluids." The theory of centres of force attracting at a
distance was no doubt founded on the effects known to be produced by the heavenly bodies on each other. It,
however, does not apply to molecular action.
For some fifty years after Faraday's brilliant discoverj'^ of magneto-induction, attention was almost exclusively
directed to the production of electric currents in one direction instead of interrupted or to-and-fro currents, such
as are obtained by the rapid alternate thrusting and withdrawing of the north pole of a magnet into a spool of
insulated wire. The interrupted currents are, however, the more varied and powerful, especially when the hnes
of magnetic force are made to quiver at high speeds. They can by means of a transformer convert a comparatively
low electric pressure or voltage into an enormously high one. They can produce electric hght, heat wires, and
accomplish most, if not all, the work performed by the constant currents. The interrupted currents are daily
becoming more important, from the great variety of work they are capable of performing.
In order to convert interrupted or to-and-fro electric currents into constant currents flowing in one direction,
a commutator is necessary. This is to the electric circuit what the eccentric is to the valves of the steam-engine
when it lets on and cuts ofE the steam which gives continuity of movement to the piston and fly-wheel. The com-
mutator is briefly a mechanical arrangement by the aid of which the free ends or terminals (electrodes) of wires
conveying interrupted or alternating currents of electricity are transposed or crossed, with the result that the
interrupted currents are converted into constant currents.
Electric currents are pecuhar in this : if the currents in two neighbouring wires are running in the same direc-
tion, the wires attract each other ; if in opposite directions, they repel each other. In the case of magnetic poles,
like poles repel and unUke poles attract.
The production of electricity is not less interesting than its mysterious manifestations. It occurs as a natural
product in the lodestone, the lightning, and in certain fishes ; notably the electric eel of tropical America, the
electric rays, and the electric cat or sheath-fish.
In the fishes, electricity is manufactured by special glands, and discharged in self-defence or attack. The
discharge results in a shock hke that obtained from ordinary electricity or from lightning.^ In the case of
the electric eel, according to Humboldt, the discharge is sufficiently strong to destroy a horse or other large
animal.
ATMOSPHERIC AND OTHER ELECTRICITY
The mode of production of atmospheric electricity is still svb judice. It has been ascertained not to be due,
as was supposed, to the evaporation of water, although the friction of the particles of water against material
substances can produce it. Thus, in Armstrong's electrical machine, where jets of spray are forced through nozzles,
a high state of electrification is obtained. The friction of dust particles is also a potent cause of electrification ;
the tops of the Egyptian pyramids during sand-storms being strongly charged.
Electricity is produced artificially in many ways—for example, by the action of certain fluids on different metals,
as in galvanic batteries—that is, when two dissimilar metals are connected by a wire and immersed in a
by heat, when two dissimilar metals are mechanically joined and heat is applied
liquid that acts upon one of them ;
to the junction ; by friction, when a warm glass rod is rubbed with dry silk. The most common form of artificial
electricity is that produced by the aid of the dynamo machine, which is driven by steam-engines, petrol-engines,
or water running at high velocities. The modern dynamo consists of a number of wire coils arranged on a
rapidly revolving shaft surrounded by fixed pieces of iron, around which the currents formed in the revolving coils
are made to circulate. The speed of the shaft, in some cases, exceeds one thousand revolutions per minute.
" Currents of electricity are excited in a coil of wire passing rapidly near the poles of a magnet ; a current being
excited in the coil in one direction by movement near a south pole, and in the opposite direction by movement
near a north pole ; a commutator can be made to direct these opposite currents in the same direction through
another coil." The electricity generated by the dynamo is carried away by wires, and used up or stored as desired.
The electricity flows so long as the dynamo is at work, and is delivered at the extremities or electrodes of the wire.
' According to Trowbridge the bolt of lightning is not one continuous discharge, but is an interrupted, alternating current which pulsates
to and fro ten or twelve times, or even more, in a millionth of a second. Photographs of powerful electric sparks lead one to conclude that a
discharge of lightning makes way for its oscillations by first breaking down the resistance of the air by means of a disruptive pilot spark ; through
the hole thus made in the air the subsequent surgings or oscillations take place. In lightning discharges high electro-motive force and great
quantity are frequently combined in a very short interval of time. The study of the disruptive or oscillatory discharge of lightning is closely
related to that of the bnish discharge and the phenomenon of the aurora borealis.