A Catechism of Familiar Things | Page 4

shock is very
powerful, but is only accompanied by light when the fluid is obstructed
in its passage. The production and condensation of vapor is a great
source of the atmospheric electricity.
Condensation, the act of making any body dense or compact; that is, of
bringing its parts into closer union.

In what other sense is the term Electricity employed?
This term is also employed to designate that important branch of
knowledge which relates to the properties shown by certain bodies
when rubbed against, or otherwise brought in contact with, each other,
to attract substances, and emit sparks of fire.
Designate, to point out by some particular token.
Emit, to send forth, to throw out.
[Illustration: CUTTING AND GATHERING ICE, ON THE HUDSON
RIVER, NEW YORK.]
Whence is the word derived?
From electron, the Greek word for amber, a yellow transparent
substance, remarkable for its electrical power when rubbed: amber is of
a resinous nature, and is collected from the sea-shore, or dug from the
earth, in many parts of the world. It is employed in the manufacture of
beads and other toys, on account of its transparency; is of some use in
medicine, and in the making of varnishes.
Transparent, clear, capable of being seen through.
Resinous, containing resin, a gummy vegetable juice.
Name a few substances possessing this remarkable property.
Silks of all kinds; the hair and fur of animals, paper, sulphur, and some
other minerals; most of the precious stones; the paste of which false
gems are made; and many other substances used by us in the common
affairs of life, are susceptible of electrical excitement; among domestic
animals the cat furnishes a remarkable instance. When dry and warm,
the back of almost any full-grown cat (the darker its color the better)
can be excited by rubbing it with the hand in the direction of the hair, a
process which is accompanied with a slight snapping noise, and in the
dark by flashes of pale blue light. When a piece of glass is rubbed with

silk, or a stick of red sealing-wax with woollen cloth, each substance
acquires the property of attracting and repelling feathers, straws,
threads of cotton, and other light substances; the substances just
mentioned as highly electric are, however, merely specimens. All
objects, without exception, most probably are capable of being
electrically excited; but some require more complicated contrivances to
produce it than others.
Electric, having the properties of electricity.
Susceptible, disposed to admit easily.
Repelling, the act of driving back.
Complicated, formed by the union of several parts in one.
Is there not a machine by which we are enabled to obtain large supplies
of electric power at pleasure?
Yes; the electrical machine. It is made of different forms and sizes: for
common purposes those of the simplest form are the best. A common
form of the machine consists of a circular plate of glass, which can be
turned about a horizontal axis by means of a suitable handle. This plate
turns between two supports, and near its upper and lower edges are two
pairs of cushions, usually made of leather, stuffed with horse-hair and
coated with a mixture of zinc, tin, and mercury, called an amalgam.
These cushions are the rubbers for producing friction, and are
connected with the earth by means of a metal chain or rod. Two large
hollow cylinders of brass with globular ends, each supported by two
glass pillars, constitute the reservoir for receiving the electricity. They
are called the prime conductors, and are supplied with U-shaped rods
of metal, furnished with points along their sides, called combs, for the
purpose of receiving the electricity from the glass plate, the arms of the
U being held upon either side. The other ends of the conductors are
connected by a rod from the middle of which projects another rod
terminating in a knob, for delivering the spark.
On turning the plate, a faint snapping sound is heard, and when the

room is darkened, a spark is seen to be thrown out from the knob
projecting from the prime conductors.
Many curious and interesting experiments may be performed by means
of the machine, illustrating the general properties of electricity. For
instance: a person standing on an insulated bench, that is, a bench with
glass legs, or having the legs resting on glass, and having one hand on
the conductor, can send sparks, with the other hand, to everything and
everybody about. This illustrates communication of electricity by
contact. A wooden head, covered with long hairs, when placed on the
conductor, illustrates electrical repulsion, by the hairs standing on end.
If the hand is held to the knob, sparks will pass from it in rapid
succession, causing in the hand a sensation of pain. This is called an
electric shock, and is caused by