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VIII. A Dynamical Theory of the Electromagnetic Field. By J. Clerk Maxwell, F.B.S. 
Received October 27,; — Read December 8, 1864. 
PART I. — INTRODUCTORY. 
(1) The most obvious mechanical phenomenon in electrical and magnetical experiments 
is the mutual action by which bodies in certain states set each other in motion while 
still at a sensible distance from each other. The first step, therefore, in reducing these 
phenomena into scientific form, is to ascertain the magnitude and direction of the force 
acting between the bodies, and when it is found that this force depends in a certain 
way upon the relative position of the bodies and on their electric or magnetic condition, 
it seems at first sight natural to explain the facts by assuming the existence of some- 
thing either at rest or in motion in each body, constituting its electric or magnetic state, 
and capable of acting at a distance according to mathematical laws. 
In this way mathematical theories of statical electricity, of magnetism, of the mecha- 
nical action between conductors carrying currents, and of the induction of currents have 
been formed. In these theories the force acting between the two bodies is treated with 
reference only to the condition of the bodies and their relative position, and without 
any express consideration of the surrounding medium. 
These theories assume, more or less explicitly, the existence of substances the parti- 
cles [of which have the property of acting on one another at a distance by attraction 
or repulsion. The most complete development of a theory of this kind is that of 
M. W. Weber*, who has made the same theory include electrostatic and electromagnetic 
phenomena. 
In doing so, however, he has found it necessary to assume that the force between 
two electric particles depends on their relative velocity, as well as on their distance. 
This theory, as developed by MM. W. Weber and C. Neumann!, is exceedingly 
ingenious, and wonderfully comprehensive in its application to the phenomena of 
statical electricity, electromagnetic attractions, induction of currents and diamagnetic 
phenomena; and it comes to us with the more authority, as it has served to guide the 
speculations of one who has made so great an advance in the practical part of electric 
science, both by introducing a consistent system of units in electrical measurement, and 
by actually determining electrical quantities with an accuracy hitherto unknown. 
* Electrodynamiscbe Maassbestimmimgen. Leipzic Trans, vol. i. 1849, and Taylor’s Scientific Memoirs, vol. v. 
art. xiv. 
f “ Explicare tentatur quomodo fiat ut lucis planum polarizationis per vires electricas vel magneticas decli- 
netur.” — Halis Saxonum, 1858; 
MDCCCLXV. 3 R 
