806 Dr. Faraday's Researches in Electricity. [Series xix. 



rent 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 per- 

 pendicular 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 expressing 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 re- 

 present 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 de- 

 gree 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 sub- 

 jected 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 ; 



