Text extracted from opening pages of book: SCIENTIFIC MEMOIRS
EDITED BY J. S. AMES, PH. D. PROFESSOR OP PHYSICS IN JOHNS HOPKINS
UNIVERSITY VIII. THE EFFECTS OF A MAGNETIC FIELD ON RADIATION THE
EFFECTS OF A MAGNETIC FIELD ON RADIATION MEMOIRS BY FARADAY, KERR
AND ZEEMAN EDITED BY E. P. LEWIS, PH. D. NEW YORK .: CINCINNATI:
CHICAGO AMERICAN BOOK COMPANY COPYRIGHT, 1900, BY AMERICAN BOOK
COMPANY. W. P. I PBEFACE Historical IN the early part of this
century possible relationships between the various forces of nature
began to attract the attention of physicists. In 1800 William
Herschel discovered that a heat spectrum is superimposed on and
extends be yond the visible solar spectrum, indicating some
relationship between heat and light. This seems to have suggested
to Domenico Morichini, of Kome, the search for a relationship
between light and magnetism. In 1812 he claimed that he had been
able to magnetize steel needles by exposing them to the violet
radiation in the solar spectrum. Others, including Mrs. Somemlle,
in England, believed that they had verified his results, but many
wore unable to reproduce them, and it was finally demonstrated that
all these effects had been due to other causes. The dispute over
this question extended over many years, and is an instructive
illustration of the difficulty which even skilled experimenters may
have in solving a com paratively simple experimental problem. About
1825 Sir John Herschel sent a polarized beam of light along the
axis of a helix carrying an electric current. Exami nation with an
analyzer showed no effect. He also intended to test the effect of a
polarized beam passing tangentially by a con ductor carrying a
current, but never executedthe experiment. No other attempt to show
a relationship between light and magnetism seems to have been made
until Faraday undertook the investigation described in the
following pages. Theoretical In the Proceedings of the Royal
Society for June, 1856, Sir William Thomson wrote: The magnetic
influence on light PREFACE discovered by Faraday depends on the
direction of motion of moving particles. For instance, in a medium
possessing it, particles in a straight line parallel to the lines
of magnetic force, displaced to a helix round this line as axis,
and then projected tangentially with such velocities as to describe
circles, will have different velocities according as their mo tions
are round in one direction ( the same as the nominal direction of
the galvanic current in the magnetizing coil) or in the contrary
direction. But the elastic reaction of the medium, must be the same
for the same displacements, what ever be the velocities ami
directions of the particles; that is to say, the forces which are
balanced by centrifugal force of the circular motions are equal,
while the luminiferous motions are unequal. The absolute circular
motions being, therefore, either equal, or such as to transmit
equal centrifugal forces to the particles initially considered, it
follows that the luminife rous motions are only components of the
whole motion; ami that a less luminiferous component in one
direction, com pounded with a motion existing in the medium when
trans mitting no light, gives an equal resultant to that of a
greater luminiferous motion in the contrary direction, compounded
with the same non-luminous motion. Maxwell, in his Electricity and
Magnetism, , vol. ii., chap, xxi, offers thefollowing partial
physical explanation as an exten sion of the above remarks: * It is
a well-known theorem in kinematics that two uniform circular
vibrations, of the same amplitude, having the same periodic time,
and in the same plane, but revolving in opposite directions, are
equivalent, when compounded together, to a rectilinear vibration.
The periodic time of this vibration is equal to that of the
circular vibrations, its amplitude is double, and its direction is
in the line joining the points at which two particles, describing
the circular vibrations in opposite dire
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