2 
ME. J. E. H. GOEDON ON THE DETEEMINATION OF 
INTEODUCTOEY. 
In the year 1845 Faraday discovered that if plane polarized light passes through 
certain media, and these media be acted on by a sufficiently powerful magnetic force, 
the. plane of polarization is rotated. 
About the year 1853 M. Verdet commenced a long and exhaustive examination of 
the subject, and his first result was published in Ann. de Chimie et de Phys. 3 ser. tom. xli. 
In Maxwell’s 4 Electricity,’ Art. 808, vol. ii. p. 400, M. Yerdet’s results are sum- 
marized as follows : — 
The angle through which the plane of polarization is turned is proportional 
(1) To the distance which the ray travels within the medium ; 
(2) To the intensity of the resolved part of the magnetic force in the direction of 
the ray. 
(3) The amount of rotation depends on the nature of the medium. 
Prof. Clerk Maxwell then goes on to say 
“ These three statements are included in the more general one that the angular 
rotation is numerically equal to the amount by which the magnetic potential increases 
from the point at which the ray enters the medium to that at which it leaves it multi- 
plied by a coefficient which for diamagnetic media is generally positive.” 
The object of the present research is to determine the coefficient for a particular 
medium and for light of a particular wave-length. 
“ Verdet’s constant ” is this coefficient for a particular medium and ray, or, in other 
words, it is the angular rotation produced on that ray and in that medium by a difference 
of magnetic potential equal to unity. 
In order that the measurements may be expressed in absolute units, it is necessary 
to modify M. Verdet’s mode of proceeding in several respects. 
In particular an electromagnet with an iron core is unsuitable for this investigation, 
for both the amount and the distribution of the magnetic force between the poles 
depend on the properties of the iron core, and cannot be deduced from the strength of 
the current in the helix. Faraday’s heavy glass and other media having the highest 
power of rotating the plane of polarization were also unsuitable to be used as standard 
media on account of the difficulty of procuring specimens exactly alike. The following 
method was therefore adopted:— 
The magnetic force was produced by means of an electric current in a helix without 
an iron core, and bisulphide of carbon enclosed in a tube with glass ends placed within 
the helix was chosen as the medium. The constants of the helix were measured by 
the methods described in pages 4-14. 
The strength of the current in it was deduced from the deflection of a small magnet 
bisulphide of carbon instead of water, which was used in the earlier experiments, thus increasing the rotation, 
and have made the helix act as its own galvanometer by suspending a magnetized needle in its neighbourhood. 
In consequence of these improvements in the methods, I have, through Prof. Maxwell, requested the Council 
of the Eoyal Society to permit me to withdraw my first Memoir, and have at the same time embodied in the 
present paper those portions of the first which are necessary for a due understanding of the second. 
