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



itself of their force. Thus, when the two contrary poles were 

 on one side of the polarized ray (2150.), and the heavy glass 

 in its best position between them and in the ray (2152.), the 

 bringing of a large piece of iron near to the glass on the other 

 side of the ray, caused the power of the diamagnetic to fall. 

 This was because certain lines of magnetic force, which at first 

 passed through the glass parallel to the ray, now crossed the 

 glass and the ray ; the iron giving two contrary poles oppo- 

 site the poles of the magnet, and thus determining a new 

 course for a certain portion of the magnetic power, and that 

 across the polarized ray. 



2169. Or, if the iron, instead of being applied on the oppo- 

 site side of the glass, were applied on the same side with the 

 magnet, either near it or in contact with it, then, again, the 

 power of the diamagnetic fell, simply because the power of 

 the magnet was diverted from it into a new direction. These 

 effects depend much of course on the intensity and power of 

 the magnet, and on the size and softness of the iron. 



2170. The electro-helices (2190.) without the iron cores 

 were very feeble in power, and indeed hardly sensible in their 

 effect. With the iron cores they were powerful, though no 

 more electricity was then passing through the coils than be- 

 fore (1071.). This shows, in a very simple manner, that the 

 phsenomena exhibited by light under these circumstances, is 

 directly connected with the magnetic form of force supplied 

 by the arrangement. Another effect which occurred illus- 

 trated the same point. When the contact at the voltaic bat- 

 tery is made, and the current sent round the electro-magnet, 

 the image produced by the rotation of the polarized ray does 

 not rise up to its full lustre immediately, but increases for a 

 couple of seconds, gradually acquiring its greatest intensity; 

 on breaking the contact, it sinks instantly and disappears ap- 

 parently at once. The gradual rise in brightness is due to 

 the time which the iron core of the magnet requires to evolve 

 all that magnetic power which the electric current can deve- 

 lope in it; and as the magnetism rises in intensity, so does its 

 effect on the light increase in power; hence the progressive 

 condition of the rotation. 



2171. I cannot as yet find that the heavy glass (2151.), 

 when in this state, i. e. with magnetic lines of force passing 

 through it, exhibits any increased degree, or has any specific 

 magneto-inductive action of the recognized kind. I have 

 placed it in large quantities, and in different positions, between 

 magnets and magnetic needles, having at the time very deli- 

 cate means of appreciating any difference between it and air, 

 but could find none. 



