Encyclopaedia Britannica > Volume 8, DIA-England

574
History.
Discove¬
ries of x\m
pere.
ELECTRICITY.
Discove¬
ries of Ara^
go, Davy,
and See-
beck.
Discove¬
ries of M.
Savary.
Discove-
ries of Pro'
fessor Er-
man.
Discove¬
ries of Dr
Seebeck,
1822.
the experiment, he discovered the fundamental law, that
the magnetical effect of the Voltaic current had a circular
motion round the current, or round the conductor, or the
wire through which the current passed. M. Ampere
of Paris soon afterwards made the important discovery,
that two wires conducting electrical currents, when sus¬
pended so as to be capable of motion, attracted each other
when the currents moved in the same direction, and re¬
pelled each other when they moved in opposite directions ;
or, to express the fact more simply, two points of electrical
currents repel each other by their similar sides, and attract
each other by their opposite sides ; so that, as Professor Oer¬
sted remarks, an electric current contains a revolving ac¬
tion, exhibiting every appearance of polarity.
In 1820 M. Arago, Sir H. Davy, and Dr Seebeck of
1 Berlin, without being acquainted with each other’s labours,
discovered the power of the electric current to impart
magnetism to iron and steel needles; but the most singu¬
lar discovery on this branch of the subject was made by
M. Savary, who found that small steel needles placed at
different but very short distances from a wire conducting
an electrical current, are magnetised in different directions.
Needles in contact with the wire are magnetised in the
usual or positive direction; while needles at the distance
of 1*1 millimeter, or ^th of an inch, are magnetised in an
opposite direction, which he calls negative. At the dis¬
tance of two inches from the wire there was a neutral line
in which the needles wrere not magnetised at all. When
the distance of the unmagnetised needle wus increased from
three to eight millimeters it again became positively mag¬
netic, the maximum effect taking place at the distance
of 5^ millimeters. Between the distance of 8-6 and 2P4
millimeters the magnetism was a second time negative,
the effect increasing from 8-6 to IPG, and again reaching
the vertical or zero point at 2P4'. Beyond the distance
of twenty-three millimeters the magnetism was again po¬
sitive. With different conducting wires M. Savary found,
that within certain limits the maximum intensity is pro¬
duced at a greater distance from the wire, and the num¬
ber of alternations of positive and negative direction is
also greater in proportion, as the wire is shorter in pro¬
portion to the length of the helix. When needles are
placed parallel to the axis of a helix of narrow windings,
they all receive the same kind of magnetism; but when
the electrical charge is increased from one jar to a bat¬
tery of twenty-two superficial feet, six alternations, viz.
three positive and three negative, are obtained. When
Voltaic electricity is substituted for ordinary electricity,
the alternations are destroyed by a continued current, but
appear when the current is established only for an instant.
These curious experiments were followed by those of
Professor Erman of Berlin, who found that when an elec¬
trical discharge passes through the centre of a circular
disc of steel, and in a line perpendicular to its surface,
no apparent magnetism is developed; but when a slit is
made in the plate, or a sector cut out of it, the side of
the disc opposite to the slit, or the sectoral opening, ex¬
hibits the opposite magnetism. MM. Gay Lussac and
Welther obtained the same result with a steel ring.
The discovery of thermo-electricity by Dr Seebeck in
1822 gave a new impulse to this branch of science. In
studying the influence of heat in Galvanic arrangements,
he was led to believe that magnetism might be developed
in two metals forming a circuit when the equilibrium of
heat in them was disturbed. He accordingly joined a se¬
micircular piece of bismuth with a similar piece of copper,
so as to form a circle by their union ; and when one of the
city b;
Farad
junctions was heated an electrical current was produced, Histc
which could show its existence only by the magnetic
needle, and which exhibited all the magnetical properties
of an electrical current.
In the same year in which Dr Seebeck made this re-Dr ira
markable discovery, the rotation of a magnetical needle day’s (
round an electrical current, and of a body transmittingcoverii
an electrical current round a magnet, were exhibited in a
series of beautiful and highly ingenious experiments by Dr
Faraday, whose subsequent discoveries place him at the
head of the cultivators of this most interesting science.
These experiments were followed by those of Arago, Exper
Barlow, Seebeck, Herschel, and Babbage, in which a re-ments
volving plate of copper gives a rotatory motion to a mag-Arao°
netic needle conveniently suspended ; but notwithstanding others
th% ingenuity and talent with which this subject was treat¬
ed by these eminent individuals, it is to Dr Faraday that
we owe a complete analysis and explanation of this curi¬
ous phenomenon.
This explanation was founded on the great discovery Disco'
of the evolution of electricity from magnetism, by which of maj
Dr Faraday laid the foundation of the new science of mag-to’elei
neto-electricity. By means of a series of simple and beau
tiful experiments with the celebrated magnets of Dr God
win Knight, and with the powerful magnet of Professor Da
niel, Dr Faraday obtained the most unequivocal and strik
ing electrical effects, though the intensity of the electri
city was very feeble, and its quantity small. He obtain
ed a distinct though feeble spark; he succeeded in con
vulsing the limbs of a frog by means of a magnet; and he
perceived also the sensation on the tongue and the flash
before the eyes, but he could not effect chemical decom¬
position by magnetism. Besides obtaining these import¬
ant results, Dr Faraday has clearly established the laws
according to which a magnet developes magnetic cur¬
rents. Fie applies these laws to the explanation of the
reciprocal action of revolving magnets and metals, and he
adduces unquestionable proofs of the production of electri¬
city by terrestrial magnetism.
These important results have been more recently ex¬
tended by Dr Faraday and others. M. Pixii observed
attractions and repulsions in the electricity evolved by
magneto-electric induction; and by an ingenious and
powerful apparatus he obtained a great degree of diver¬
gence in the gold leaves of an electrometer. At the
meeting of the British Association at Oxford in June
1832, Dr Faraday, by means of Mr Snow Harris’s electro¬
meter, subsequently described, succeeded in heating a
wire by magneto-electric induction. By means of the
magneto-electric apparatus of M. Pixii already referred
to, he and M. Hachette decomposed water, and obtained
the oxygen and hydrogen in separate tubes.
In the progress of his electrical researches, Dr Faraday ident
found it necessary for their further prosecution to esta-thevi
blish either the identity or the distinction of the electri-ylectr
cities excited by different means ; and in a paper of great ^
value, he has established beyond a doubt the identity ofpr j>,
common electricity, Voltaic electricity, magneto-electri-
city, thermo-electricity, and animal electricity. The phe¬
nomena exhibited in these five kinds of electricity do not
differ in kind, but merely in degree; and in this respect
they vary in proportion to the variable circumstances of quan¬
tity and intensity, which can at pleasure be made to change
in almost any one of the kinds of electricity, as much as it
does between one hind and another. Dr Faraday has given
the following interesting table of the experimental effects
common to the electricities derived from different sources.
1 The cross indicates that the effect at the top of this table is produced by the electricity mentioned in the column at the side.