ROTATION OF A RAY OF LIGHT BY ELECTRIC FORCE. 
11 
tion, I will assume, as is usually done, that the current passes from the zinc through 
the acid to the platinum in the same cell (663. 667. 1627.) : if such a current pass 
under the ray towards the right, upwards on its right side, and over the ray towards 
the left, it will give left-handed rotation to it ; or, if the current pass over the ray to 
the right, down on the right side, and under it towards the left, it will induce it to 
rotate to the right-hand. 
2199. The law, therefore, by which an electric current acts on a ray of light is 
easily expressed. When an electric current passes round a ray of polarized light in 
a plane perpendicular to the ray, it causes the ray to revolve on its axis, as long as 
it is under the influence of the current, in the same direction as that in which the 
current is passing. 
2200. The simplicity of this law, and its identity with that given before, as ex- 
pressing the action of magnetism on light (2160.), is very beautiful. A model is not 
wanted to assist the memory; but if that already described (2161.) be looked at, the 
line round it will express at the same time the direction both of the current and the 
rotation. It will indeed do much more ; for if the cylinder be considered as a piece 
of iron, and not a piece of glass or other diamagnetic, placed between the two poles N 
and S, then the line round it will represent the direction of the currents, which, 
according to Ampere’s theory, are moving round its particles ; or if it be considered 
as a core of iron (in place of a core of water), having an electric current running 
round it in the direction of the line, it will also represent such a magnet as would 
be formed if it were placed between the poles whose marks are affixed to its ends. 
2201. I will now notice certain points respecting the degree of this action under 
different circumstances. By using a tube of water (2194.) as long as the helix, but 
placing it so that more or less of the tube projected at either end of the helix, I was 
able, in some degree, to ascertain the effect of length of the diamagnetic, the force of 
the helix and current remaining the same. The greater the column of water subjected 
to the action of the helix, the greater was the rotation of the polarized ray ; and the 
amount of rotation seemed to be directly proportionate to the length of fluid round 
which the electric current passed. 
2202. A short tube of water, or a piece of heavy glass, being placed in the axis of 
the Woolwich helix (2192.), seemed to produce equal effect on the ray of light, 
whether it were in the middle of the helix or at either end ; provided it was always 
within the helix and in the line of the axis. From this it would appear that every 
part of the helix has the same effect ; and, that by using long helices, substances 
may be submitted to this kind of examination which could not be placed in sufficient 
length between the poles of magnets (2150.). 
2203. A tube of water as long as the Woolwich helix (2192.), but only 0*4 of an 
inch in diameter, was placed in the helix parallel to the axis, but sometimes in the 
axis and sometimes near the side. No apparent difference was produced in these dif- 
ferent situations ; and I am inclined to believe (without being quite sure) that the 
c 2 
