28 ELEMENTS OF STATIC ELECTRICITY. 



ultaneous evolution of heat, light, and electricity from 

 the same causes. 



In the thermo-electric battery we have an example 

 of the direct production of electricity by heat. 



POLARIZED LIGHT AND ELECTKICLTY. Experiments 

 with polarized light, made by Faraday and others, fur- 

 nish strong evidence of the close alliance, if not actual 

 identity, of light and electricity. 



It is known that light, from certain peculiarities of 

 reflection and transmission, undergoes a change, so 

 that when subsequently transmitted, its action is dif- 

 ferent from that of the original transmission, and this 

 change has been termed polarization. 



Let a plate of tourmaline be so placed that a ray of 

 light falling on it shall be transmitted at right angles 

 to a particular direction within the crystal, known as 

 its optical axis. Then let another tourmaline plate be 

 so placed with reference to this one that their optical 

 axes are parallel, and that the ray shall pass through 

 both and form a bright spot on a screen beyond. 



Now let either plate be turned, so that their optical' 

 axes are no longer parallel to each other, but still at 

 right angles to the ray ; the bright spot on the screen 

 will fade as the angle increases, till at 90 degrees it will 

 disappear. Continuing the rotation, it will re-appear, 

 increasing in brightness, till, at 180 degrees, it is en- 

 tirely restored ; then fading again till extinguished 

 at the end of the third quadrant, and again increasing 

 in brightness till restored at the end of the fourth 

 quadrant, or original position. 



This alternation of brightness and extinction de- 

 pends on the relative angular position of the plates. 

 Let either of them be turned in either direction, and 



