New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

464
MAGNETO-OPTICS
Meteorological Congress at Chicago (Bull. No. 11, Part II. U.S.
Department of Agriculture. Washington, 1895).—Repertorium
fiir Meteorologie. St Petersburg, vols. i.-xvii. — Terrestrial
Magnetism: an International Quarterly Journal. Editor, L. A.
Bauer (vol. i. 1896, and subsequent volumes).—Meteorologische
Zeitschrift. Vienna.—British Association’s Committee for the
Comparison and Reduction of Magnetic Observations. Annual
Reports.—Annales de VOhservatoire Physique Central (I. Partie).
St Petersburg.—Annales du Bureau Central MtUorologique de
France (T. 1, Memoires). Paris.—U.S. Coast and Geodetic Sur¬
vey’s Reports. Washington.—Also publications of individual
magnetic observatories, more especially of Kew, Greenwich,
Copenhagen, Utrecht, Potsdam, Vienna, Pola, Lisbon, Coimbra,
Nice, Tiflis, Colaba (Bombay), Mauritius, Batavia, Manila, Hong
Kong, Zi-ka-wei (China), Melbourne, Washington, Toronto.
(c. Ch.)
Magneto-Optics.—The first relation between
magnetism and light was discovered by Faraday,1 who
proved that the plane of polarization of a ray of light
was rotated when the ray travelled, through certain
substances, parallel to the lines of magnetic force. This
power of rotating the plane of polarization in a magnetic
field has been shown to be possessed by all refracting
substances, whether they are in the solid, liquid, or
gaseous state. The rotation by gases was established
independently by H. Becquerel,2 and Kundt and Rdntgen,3
while Kundt4 found that films of the magnetic metals,
iron, cobalt, nickel, thin enough to be transparent, produced
enormous rotations, these being in iron and cobalt mag¬
netized to saturation at the rate of 200,000° per cm. of
thickness, and in nickel about 89,000°. The direction of
rotation is not the same in all bodies. If we call the rota¬
tion positive when it is related to the direction of the mag¬
netic force, like rotation and translation in a right-handed
screw, or, what is equivalent, when it is in the direction
of the electric currents which would produce a magnetic
field in the same direction as that which produces the
rotation, then most substances produce positive rotation.
Among those that produce negative rotation are ferrous
and ferric salts, ferri-cyanide of potassium, the salts of
lanthanum, cerium and didymium, and chloride of titanium.5
For slightly magnetizable substances the amount of rotation in a
space PQ is proportional to the difference between the magnetic
potential at P and Q ; or if 0 is the rotation in PQ, flPi Gq, the
magnetic potential at P and Q, then
0 = R(Op —fig),
where R is a constant, called Verdet’s constant, which depends
upon the refracting substance, the wave-length of the light, and
the temperature. The following are the values of R (when the
rotation is expressed in circular measure) for the D line and a
temperature of 18° C. :—
Substance.
Carbon-bisulphide.
Water .
Alcohol....
Ether . . . .
Oxygen (at 1 atmosphere)
Faraday’s heavy glass
B x 10 5.
1-222
1-225
•377
•3808
•330
•315
•000179
1-738
Observer.
Lord Rayleigh 6 and Kbpsel.7
Rodger and Watson.8
Arons.9
Rodger and Watson.8
Du Bois.10
Du Bois.10
Kundt and Rontgen (7.C.).
The variation of Verdet’s constant with temperature has been
determined for carbon-bisulphide and water by Rodger and Watson
{l. c.). They find if R*, R0 are the values of Verdet’s constant at
t°C. and 0°C. respectively, then for CS2 R4=R0 (1- '0016961), and
for H20 R* = R0 (1 - -00003051; - •00000305!;2).
For the magnetic metals Kundt found that the rotation did not
increase so rapidly as the magnetic force, but that as this force was
increased the rotation reached a maximum value. This suggests
that the rotation is proportional to the intensity of magnetization,
and not to the magnetic force.
The amount of rotation in a given field depends greatly upon the
wave-length of the light; the shorter the wave-length the greater the
rotation, the rotation varying a little more rapidly than the inverse
square of the wave-length. Verdet11 has compared in the cases of
carbon bisulphide and creosote the rotation given by the formula
„ c2 / di\
e=m->vV-xd\)
with those actually observed; in this formula 6 is the angular
rotation of the plane of polarization, m a constant depending on
the medium, X the wave-length of the light in air, and i its index
of refraction in the medium. Verdet found that though the
agreement is fair, the differences are greater than can be explained
by errors of experiment.
Verdet12 has shown that the rotation of a salt solution
is the sum of the rotations due to the salt and the solvent;
thus, by mixing a salt which produces negative rotation
with water which produces positive rotation, it is possible
to get a solution which does not exhibit any rotation.
Such solutions are not in general magnetically neutral.
By mixing diamagnetic and paramagnetic substances we
can get magnetically neutral solutions, which, however,
produce a finite rotation of the plane of polarization.
The relation of the magnetic rotation to chemical consti¬
tution has been studied in great detail by Perkin,13 Wachs-
muth,14 Jahn,15 and Schonrock.16
The rotation of the plane of polarization may con¬
veniently be regarded as denoting that the velocity of
propagation of circular-polarized light travelling along the
lines of magnetic force depends upon the direction of
rotation of the ray, the velocity when the rotation is
related to the direction of the magnetic force, like rotation
and translation on a right-handed screw being different
from that for a left-handed rotation. A plane-polarized
ray may be regarded as compounded of two oppositely
circularly-polarized rays, and as these travel along the lines
of magnetic force with different velocities, the one will gain
or lose in phase on the other, so that when they are again
compounded they will correspond to a plane-polarized ray,
but in consequence of the change of phase the plane of
polarization will not coincide with its original position.
Reflection from a Magnet.—Kerr17 in 1877 found that
when plane-polarized light is incident on the pole of an
electromagnet, polished so as to act like a mirror, the
plane of polarization of the reflected light is rotated by
the magnet. Further experiments on this phenomenon have
been made by Bighi,18 Kundt,19 Du Bois,20 Sissingh,21
Hall,22 Hurion,23 Kaz,24 and Zeeman.25 The simplest case is
when the incident plane-polarized light falls normally on
the pole of an electromagnet. When the magnet is not
excited the reflected ray is plane - polarized; when the
magnet is excited the plane of polarization is rotated
through a small angle, the direction of rotation being
opposite to that of the currents exciting the pole. Righi
found that the reflected light was slightly elliptically
polarized, the axes of the ellipse being of very unequal
magnitude. A piece of gold-leaf placed over the pole
entirely stops the rotation, showing that it is not produced
in the air near the pole. Rotation takes place from
magnetized nickel and cobalt as well as from iron, and is
in the same direction (Hall). Righi has shown that the
rotation at reflection is greater for long waves than for
short, whereas, as we have seen, the Faraday rotation is
greater for short waves than for long. The rotation for
different coloured light from iron, nickel, cobalt, and
magnetite has been measured by Du Bois; in magnetite
the direction of rotation is opposite to that of the other
metals. When the light is incident obliquely and not
normally on the polished pole of an electromagnet, it is
elliptically polarized after reflection, even when the plane
of polarization is parallel or at right angles to the plane of
incidence. According to Righi, the amount of rotation
when the plane of polarization of the incident light is
perpendicular to the plane of incidence reaches a maxi¬
mum when the angle of incidence is between 44° and
68°, while when the light is polarized in the plane of
incidence the rotation steadily decreases as the angle of
incidence is increased. The rotation when the light is
polarized in the plane of incidence is always less than