308 Dr. Faraday's Besearches in Electricity. 



second tube was then placed over this, so that there was now 

 a thickness of iron equal to two-eighths of an inch between 

 the water and the helix; the water had more power of rota- 

 tion than before. On placing the third tube of iron over the 

 two former, the power of the water fell, but was still very con- 

 siderable. These results are complicated, being dependent 

 on the new condition which the character of iron gives to its 

 action on the forces. Up to a certain amount, by increasing 

 the development of magnetic forces, the helix and core, as a 

 whole, produce increased action on the water; but on the ad- 

 dition of more iron and the disposal of the forces through it, 

 their action is removed in part from the water and the rotation 

 is lessened. 



2210. Pieces of heavy glass (2151.), placed in iron tubes in 

 the helices, produced similar effects. 



2211. The bodies which were submitted to the action of 

 an electric current in a helix, in the manner already described, 

 were as follows: — Heavy glass (2151. 2176.), water, solution 

 of sulphate of soda, solution of tartaric acid, alcohol, aether, 

 and oil of turpentine; all of which were affected, and acted on 

 light exactly in the manner described in relation to magnetic 

 action (2173.). 



2212. I submitted air to the influence of these helices care- 

 fully and anxiously, but could not discover any trace of action 

 on the polarized ray of light. I put the long helix (2190.) 

 into the other two (2191. 2192.), and combined them all into 

 one consistent series, so as to accumulate power, but could 

 not observe any effect of them on light passing through air. 



2213. In the use of helices, it is necessary to be aware of 

 one effect, which might otherwise cause confusion and trouble. 

 At first, the wire of the long helix (2190.) was wound directly 

 upon the thin glass tube which served to contain the fluid. 

 When the electric current passed through the helix it raised 

 the temperature of the metal, and that gradually raised the 

 temperature of the glass and the film of water in contact with 

 it, and so the cylinder of water, warmer at its surface than its 

 axis, acted as a lens, gathering and sending rays of light to 

 the eye, and continuing to act for a time after the current was 

 stopped. By separating the tube of water from the helix, and 

 by other precautions, this source of confusion is easily avoided. 



2214. Another point of which the experimenter should be 

 aware, is the difficult}', and almost impossibility, of obtaining 

 a piece of glass which, especially after it is cut, does not de- 

 polarize light. When it does depolarize, difference of posi- 

 tion makes an immense difference in the appearance. By al- 

 ways referring to the parts that do not depolarize, as the black 



