[Footnote 1: Introductory to the course of Lectures on Physics at
Washington University, St. Louis, Missouri--_Kansas City
Review._]
Prof. FRANCIS E. NIPHER.
It was known six hundred years before Christ that when amber is rubbed
it acquires the power of attracting light bodies. The Greek name for
amber, _elektron_, was afterward applied to the phenomenon. It was
also known to the ancients that a certain kind of iron ore, first
found at Magnesia, in Asia Minor, had the property of attracting
iron. This phenomenon was called magnetism. This is the history of
electricity and magnetism for two thousand years, during which these
facts stood alone, like isolated mountain peaks, with summits touched
and made visible by the morning sun, while the region surrounding and
connecting them lay hidden and unexplored.
In fact, it is only in more recent times that men could be found
possessing the necessary mental qualities to insure success in
physical investigation. Some of the ancients were acute observers, and
made valuable observations in descriptive natural history. They also
observed and described phenomena which they saw around them, although
often in vague and mystical terms.
They, however, were greatly lacking in power to discriminate
between the possible and the absurd, and so old wives' tales,
acute speculations, and truthful observations are strangely jumbled
together. With rare exceptions they did not contrive new conditions to
bring about phenomena which Nature did not spontaneously exhibit--they
did not experiment. They attempted to solve the universe in their
heads, and made little progress.
In mediæval times intellectual men were busy in trying to set each
other right, and in disputing and arguing with those who believed
themselves to be right. It was an era of intellectual pugilism,
and nothing was done in physics. In fact, this frame of mind is
incompatible with any marked success in scientific work.
The physical investigator cannot take up his work in the spirit of
controversy; for the phenomena and laws of Nature will not argue with
him. He must come as a learner, and the true man of science is content
to learn, is content to lay his results before his fellows, and is
willing to profit by their criticisms. In so far as he permits himself
to assume the mental attitude of one who defends a position, in so far
does he reveal a grave disqualification for the most useful scientific
work. Scientific truth needs no man's defense, but our individual
statements of what we believe to be truth frequently need criticism.
It is hardly necessary to remark, also, that critics are of various
degrees of excellence, and it seems that those in whom the habit of
criticism has become chronic are of comparatively little service to
the world.
The great harbinger of the new era was Galileo. There had been
prophets before him, and after him came a greater one--Newton. They
did nothing of note in electricity and magnetism, but they were filled
with the true spirit of science, they introduced proper and reasonable
methods of investigation, and by their great ability and distinguished
success they have produced a revolution in the intellectual world.
Other great men had also appeared, such as Leibnitz and Huyghens; and
it became very clear that the methods of investigation which had borne
such fruit in the days of Galileo were not disposed of completely
by his unwilling recantation; it became very clear that the new
civilization which was dawning upon Europe was not destined to the
rude fate which had overwhelmed the brilliant scientific achievements
of the Spanish Moors of a half century before.
Already in 1580, about the time when Galileo entered Pisa as a
student, Borroughs had determined the variation of the magnetic needle
at London, and we have upon the screen a view of his instrument, which
seems rude enough, in comparison with the elaborate apparatus of our
times. The first great work on electricity and magnetism was the "De
Magnete" of Gilbert, physician of Queen Elizabeth, published in
1600. Galileo, already famous in Europe, recognized in the methods of
investigation used by Gilbert the ones which he had found so fruitful,
and wrote of him, "I extremely praise, admire, and envy this author."
Gilbert made many interesting contributions to magnetism, which we
shall notice in another lecture, and he also found that sulphur,
glass, wax, and other bodies share with amber the property of being
electrified by friction. He concluded that many bodies could not be
thus electrified. Gray, however, found in 1729 that these bodies were
conductors of electricity, and his discoveries and experiments were
explained and described to the president of the Royal Society while on
his death bed, and only a few hours before his death. If precautions
are taken to properly insulate conductors, all bodies which differ
in any way, either in structure, in smoothness of surface, or even in
temperature, are apparently electrified by friction. In all cases
the friction also produces heat, and if the bodies rubbed are exactly
alike, heat only is produced.
An electrified body will attract all light bodies. This gutta percha
when rubbed with a cat's skin attracts these bits of paper, and this
pith ball, and this copper ball; it moves this long lath balanced on
its center, and deflects this vertical jet of water into a beautiful
curve.
If a conductor is to be electrified, it must be supported by bad
conductors. This brass cylinder standing on a glass column has become
electrified by friction with cat's-skin. My assistant will stand upon
this insulating stool, and by stroking his hand you will observe that
with his other hand he can attract this suspended rod of wood, and you
will hear a feeble spark when I apply my knuckle to his.
Du Fay, of Paris, discovered what he called two kinds of electricity.
He found that a glass rod rubbed with silk will repel another glass
rod similarly rubbed, but that the silk would attract a rubbed
glass rod. We express the facts in the well-known law that like
electricities repel each other, and unlike attract. For a long time
the nature of the distinctions between the two electricities was not
understood. It was found later that when the two bodies are rubbed
together they become oppositely electrified, and that the two
electricities are always generated in equal quantity; so that if the
two bodies are held in contact after the rubbing has ceased the
two electricities come together again and the electrical phenomena
disappear. They have been added together, and the result is zero.
Franklin proposed to call these electricities positive and negative.
These names are well chosen, but we do not know any reason why one
should be called positive rather than the other. The electricity
generated on glass when rubbed with silk is called positive.
Let us now examine the distinction between positive and negative
electricities somewhat more closely, aiding ourselves by two cases
which are somewhat analogous.
Two air-tight cylinders, A and B, contain air at ordinary pressure.
The cylinders are connected by a tube containing an air-pump in such a
way that, when the pump is worked, air is taken from A and forced into
B. To use the language of the electricians, we at once generate two
kinds of pressure. The vessels have acquired new properties. If we
open a cock in the side of either vessel, we hear a hissing sound, if
a light body is placed before the opening in A it would be attracted,
and before the opening in B it would be repelled. Now this is only
roughly analogous to the case of the electrified bodies, but the
analogy will nevertheless aid us in our study. If the two vessels are
first connected with the air, and then closed up and the pump is set
to work, we increase the pressure in B and diminish the pressure in
A. To do this requires the expenditure of a quantity of work. If the
cylinders are connected by an open tube--a conductor--the difference
in pressure disappears by reason of a flow of gas from one vessel to
the other.
If we had a pump by means of which we could pump heat from one body
into another, starting with two bodies at the same temperature, the
temperature of one body would increase and that of the other would
diminish. If we knew less than we do of heat, we might well discuss
whether the plus sign should be applied to the heat or to the cold,
because these names were coined by people who knew very little about
the subject except that these bodies produce different sensations when
they come in contact with the human body.
Furthermore, we find that whether the hand is applied to a very hot
body or to a very cold body, the physiological effect is the same. In
each case the tissue is destroyed and a burn is produced. Shall we now
say that this burn is produced by an unusual flow of heat from the hot
body to the hand, or from the hand to the cold body, or shall we say
that it is due to an unusual flow of cold from the cold body to the
hand, or from the hand to the hot body?
Logically these expressions are identical; still we have come to
prefer one of them. It is because we have learned that in those bodies
which our fathers called hot, the particles are vibrating with greater
energy than in cold bodies, that we prefer to say that heat is added
and not cold subtracted, when a cold body becomes less cold.
Now to come back to our electrified bodies. Let us suppose that this
gutta percha, and this cat's-skin are not electrified. That means that
their electrical condition is the same as that of surrounding bodies.
Let us also suppose that their thermal condition is the same as
surrounding bodies, ourselves included--that is, they are neither hot
nor cold. We express these conditions in other words by saying
that the bodies have the same electrical _potential_ and the same
temperature.
Temperature in heat is analogous to potential in electricity. As
soon as adjacent bodies are at different temperatures, we have
the phenomena which reveal to us the existence of heat. As soon as
adjacent bodies have different electrical potentials, we have the
phenomena which reveal the existence of electricity. As soon as
adjacent regions in the air are at different pressures, we have
phenomena which reveal the existence of air.
Bodies all tend to preserve the same temperature and also the same
electrical potential. Any disturbances in electrical equilibrium are
much more quickly obliterated than in case of thermal equilibrium,
and we therefore see less of electrical phenomena than of thermal. In
thunder storms we see such disturbances, and with delicate instruments
we find them going on continuously. Changes in temperature occurring
on a large scale in our atmosphere, occurring in these gas jets,
in our fires, in the axles of machinery, and in thousands of other
places, are so familiar that we have ceased to wonder at them.
If we rub these two bodies together, the potential of the two is no
longer the same. We do not know which one has become greater, and in
this respect our knowledge of electricity is less complete than of
heat. We assume that the gutta percha has become negative. If we now
leave these bodies in contact, the potential of the cat's skin will
diminish and that of the gutta percha will increase until they have
again reached a common potential--that of the earth. As in the case of
heat and cold, we may say either that this has come about by a flow of
positive electricity from the cat's skin to the gutta percha, or by
a flow of negative electricity in the opposite direction, for these
statements are identical.
In case of our gas cylinders, the gas tends to leak out of the vessel
where the pressure is great into the vessel where it is small. The
heat tends to leak out of a body of high temperature into the colder
one, or the cold tends to go in the opposite direction. Similarly, the
plus electricity tends to flow from the body having a high potential,
to the body having a low potential, or the minus electricity tends to
go in the opposite direction.
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[ENGINEERING.]