————_ — ——_ — i 
1 
7 
, 
| 
’ 
: 
(796.) 
Mutual 
action of 
electric 
conductors, 
(797.) 
Elemen- 
tary law of 
the inverse 
square of 
the dis- 
tance, 
(798.) 
Mr Fara- 
day and De 
la Rive— 
Electro- 
magnetic 
Cuap. VIL., § 4.] 
The theory of Ampére: rejects all but push-and- 
pull forces, such as are commonly recognized in me- 
chanical physics. These forces are mutual, and be- 
long to electric currents. A permanent magnet is 
a congeries of minute parallel and circular currents, 
all acting in the same direction, which is at right 
angles to the magnetic axis or line of force. Grant- 
ing this for a moment, Oersted’s experiment shows 
that the current in the conductor acts on the currents 
in the magnet; and as a magnet places itself trans- 
versely to a conductor, the currents in the magnet 
tend to place themselves parallel to that in the con- 
ductor. Do we then find such properties in move- 
able electric conductors alone? Have they any 
mutual action?’ Does that mutual action tend to 
produce parallelism? And if so, may it be farther 
analysed into direct attractions or repulsions of the 
several parts of the electric currents upon one 
another? All these questions were answered by 
Ampeére afirmatively after due appeal to experiment. 
Two copper wires connected with voltaic circuits, 
and suspended with the requisite degree of freedom, 
approach when the currents have a similar direction, 
but are repelled when the direction is opposite in 
the two. When two moveable conductors are placed 
at right angles, or indeed at any angle, they tend to 
parallelism. All the usual phenomena of a magnet 
may be imitated by a long helix of copper wire 
through which electricity is made by some artifice’ 
continually to circulate. The position of the poles 
is the same as in a real magnet, and the name of 
pole is determined by the direction (right or left 
handed) in which the helix is wound. Such an in- 
strument, not containing one particle of iron, is at- 
tracted and repelled by a steel magnet,—obeys the 
directive influence of the earth,—gives transverse 
motion to an electric conductor near it,—in short, 
does whatever magnetized iron does. 
Thus, in the mutual action of electric currents 
(for the phenomena of static electricity are wholly 
unlike) we recognize the great discovery of Ampére. 
A new science was formed, which he called electro- 
dynamics, which he proceeded to develope with great 
skill and success. MM. Biot and Savary found that 
the electro-magnetic force exerted by an indefinite 
straight conductor and needle, varies inversely as the 
simple distance from the conductor; but looking 
to the elementary actions of each portion of the cur- 
rent, it will be found that this corresponds to the 
usual physical law of the inverse square of the dis- 
tance between the magnetic and the electric ele- 
ment, . 
Whilst Ampére was pursuing his inquiries into 
the properties of electric currents, others were vary- 
ing, in a great variety of ways, Oersted’s fundamental 
experiment. A great number of beautiful mechani- 
cal arrangements were invented, particularly by the 
elder De la Rive and by Mr Faraday. The latter, 
however, had the sole merit of effecting a most singu- 
ELECTRICITY.—OERSTED—AMPERE. 
975 
lar kind of motion, that in which a magnet float- 
ing in mercury is made to revolve continuously 
around a central conducting wire, and in like man- 
ner a conductor may be made to revolve round 
a fixed magnet; nay, stranger still, a, magnet 
acting at once as conductor and magnet, revolves 
with great velocity on its own axis when an elec- 
tric stream is made to traverse one half of its 
length. These astonishing experiments, which, in 
an earlier age, might have founded a new sect of 
astronomers and replaced the theory of Vortices, 
offered also considerable difficulties in the applica- 
tion of Ampére’s theory. They were, however, ulti- Accounted 
mately removed by Ampére himself, who analysed er Ore, 
with great skill the mechanical conditions of each 
case, and interpreting them into the language of his 
theory, showed how continuous rotations might be 
produced, according to the laws which he had esta- 
blished, by electric currents alone suitably arranged ; 
and he effected by most ingenious experimental combi- 
nations purely electro-dynamic rotations. Some other 
experiments, in which magnets seemed to produce a 
different effect from electro-dynamic cylinders, pre- 
sented a more serious obstacle, which, however, was 
removed by a rigorous demonstration of the effects 
which must ensue, if we regard the elementary mole- 
cules of a magnet as very small, and consequently the 
entire magnet as a collection of indefinitely small and 
correspondingly numerous electro-dynamic cylinders. 
By means of four critical experiments, Ampére de- 
termined completely the elementary laws of the 
mutual action of currents, including that previously 
established by Biot and Savary in the case of a mag- 
net and a conductor. This investigation was one of 
great intricacy, and was carried out with remarkable 
skill, Ampére had the field almost to himself, Savary 
making some contributions ; and, what is remarkable, 
little or nothing has been added either to the theory, 
or to the deductions from it, since his death. The 
progress of the science of electro-magnetism has 
been so astonishingly rapid since the year 1820, that 
one set of phenomena after another has for the 
time attracted almost exclusive notice. The disco- 
very of diamagnetism will probably lead to a recon- 
sideration of Ampére’s theory as applicable to all 
matter in a more general form. 
This rapid succession of interesting topics has pre- 799.) 
vented attention from being perhaps sufliciently di- Great 
rected to the importance of Ampére’s labours, a esa. 
is at least as well entitled as any other philosopher 
who has yet appeared, to be called ** the Newton of 
Electricity.” 
Ampere was of an amiable, though rather eccentric 
character. His absence of mind was proverbial, and 
his style is somewhat cumbrous and obscure, But 
he was devoted to science, the promotion of which 
was ever his first consideration, and he evidently 
himself possessed great clearness in his conceptions. 
He died on the 17th May 1836. 
His death. 
