The Story of Electricity | Page 6

can be connected together in "batteries," so as to give very
powerful effects. One method is to join the inner coat of one to the
outer coat of the next. This is known as connecting in "series," and
gives a very long spark. Another method is to join the inner coat of one
to the inner coat of the next, and similarly all the outer coats together.
This is called connecting "in parallel," or quantity, and gives a big, but
not a long spark.
Of late years the principle of induction, which is the secret of the

Leyden jar and electrophorus, has been applied in constructing
"influence" machines for generating electricity. Perhaps the most
effective of these is the Wimshurst, which we illustrate in figure 10,
where PP are two circular glass plates which rotate in opposite
directions on turning the handle. On the outer rim of each is cemented a
row of radial slips of metal at equal intervals. The slips at opposite ends
of a diameter are connected together twice during each revolution of
the plates by wire brushes S, and collecting combs TT serve to charge
the positive and negative conductors CC, which yield very powerful
sparks at the knobs K above. The given theory of this machine may be
open to question, but there can be no doubt of its wonderful
performance. A small one produces a violent spark 8 or 10 inches long
after a few turns of the handle.
The electricity of friction is so unmanageable that it has not been
applied in practice to any great extent. In 1753 Mr. Charles Morrison,
of Greenock, published the first plan of an electric telegraph in the
Scots Magazine, and proposed to charge an insulated wire at the near
end so as to make it attract printed letters of the alphabet at the far end.
Sir Francis Ronalds also invented a telegraph actuated by this kind of
electricity, but neither of these came into use. Morrison, an obscure
genius, was before his age, and Ronalds was politely informed by the
Government of his day that "telegraphs of any kind were wholly
unnecessary." Little instruments for lighting gas by means of the spark
are, however, made, and the noxious fumes of chemical and lead works
are condensed and laid by the discharge from the Wimshurst machine.
The electricity shed in the air causes the dust and smoke to adhere by
induction and settle in flakes upon the sides of the flues. Perhaps the
old remark that "smuts" or "blacks" falling to the ground on a sultry
day are a sign of thunder is traceable to a similar action.
The most important practical result of the early experiments with
frictional electricity was Benjamin Franklin's great discovery of the
identity of lightning and the electric spark. One day in June, 1792, he
went to the common at Philadelphia and flew a kite beneath a
thundercloud, taking care to insulate his body from the cord. After a
shower had wetted the string and made it a conductor, he was able to
draw sparks from it with a key and to charge a Leyden jar. The man
who had "robbed Jupiter of his thunderbolts" became celebrated

throughout the world, and lightning rods or conductors for the
protection of life and property were soon brought out. These, in their
simplest form, are tapes or stranded wires of iron or copper attached to
the walls of the building. The lower end of the conductor is soldered to
a copper plate buried in the moist subsoil, or, if the ground is rather dry,
in a pit containing coke. Sometimes it is merely soldered to the water
mains of the house. The upper end rises above the highest chimney,
turret, or spire of the edifice, and branches into points tipped with
incorrosive metal, such as platinum. It is usual to connect all the
outside metal of the house, such as the gutters and finials to the rod by
means of soldered joints, so as to form one continuous metallic network
or artery for the discharge.
When a thundercloud charged with electricity passes over the ground, it
induces a charge of an opposite kind upon it. The cloud and earth with
air between are analogous to the charged foils of the Leyden jar
separated by the glass. The two electricities of the jar, we know, attract
each other, and if the insulating glass is too weak to hold them asunder,
the spark will pierce it. Similarly, if the insulating air cannot resist the
attraction between the thundercloud and the earth, it will be ruptured by
a flash of lightning. The metal rod, however, tends to allow the two
charges of the cloud and earth to combine quietly or to shunt the
discharge past the house.

CHAPTER II