XVII
ON THE RELATIONS BETWEEN MAXWELL'S FUN-
DAMENTAL ELECTROMAGNETIC EQUATIONS
AND THE FUNDAMENTAL EQUATIONS OF THE
OPPOSING ELECTROMAGNETICS.
(Wiedemann's Annalen, 23, pp. 84-103, 1884.)
WHEN Ampère heard of Oerstedt's discovery that the electric
current sets a magnetic needle in motion, he suspected that
electric currents would exhibit moving forces between them-
selves. Clearly his train of reasoning was somewhat as
follows: The current exerts magnetic force, for a magnetic
pole moves when submitted to the action of the current; and
the current is set in motion by magnetic forces, for by the
principle of action and reaction a current-carrier will also
move under the influence of a magnet. Unless we make the
improbable assumption that different kinds of magnetic force
exist, a current-carrier must also move under the action of the
magnetic forces which a second current exerts, and thus the
interaction between currents follows.
The essential step in this reasoning is the assumption that
only one kind of magnetic force exists; that therefore the
magnetic forces exerted by currents are in all their effects
equivalent to equal and equally directed forces produced by
magnetic poles. But this assumption is well known to be
sufficient to deduce not only the existence but also the precise
magnitude of the electromagnetic actions of closed currents from
their magnetic actions. Whether Ampère actually started
from this principle or not, he certainly stated it at the close
of his investigations when he reduced the action of magnets
directly to the action of supposed closed currents.
M. P.
T
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