318 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The effect of the cotton sieve is to give a negative charge to the 

 sulphur particles and a positive charge to the red lead particles, 

 so that the sulphur particles cling to the positively charged parts 

 of the crystal and the red lead particles cling to the negatively 

 charged parts of the crystal. 



Piezo-electricity* In 1880 it was found by J. and P. Curie 

 that many kinds of crystals become electrically charged when 

 they are subjected to pressure. This effect is produced in hemi- 

 hedral crystal forms when a crystal plate with its faces at right 

 angles to the hemihedral axis is compressed between two metal 

 plates. The effect is to charge the two metal plates oppositely. 



20. Magnetic rotation of the plane of polarization of light. 

 Faraday f discovered in 1 846 that a transparent substance such 

 as glass or carbon bi-sulphide rotates the plane of polarization of 

 light when it is placed in the magnetic field and when the light is 

 passed through it in the direction of the magnetic lines of force. 



21. The Hall effect. \ When a conductor through which an 

 electric current is flowing is placed in a magnetic field, the con- 

 ductor is acted upon by a force which pushes it sidewise as ex- 

 plained in Chapter IV. Ordinarily this force does not alter the 

 distribution of current in the conductor, that is to say, the cur- 

 rent is not pushed to one side of the conductor. E. H. Hall 

 discovered in 1880, however, that the current is pushed to one 

 side of the conductor to a very slight extent in some metals, 

 especially in bismuth. This peculiar effect is satisfactorily ex- 

 plained by the electron theory of metallic conduction (see Lodge's 

 Electrons, pages 106109). 



22. The Kerr effect. One of the most universally applicable 

 principles in physics is the principle of superposition, so-called, 



* See Wiedemann, Die Lehre von der Elektricitdt, Vol. II, pages 341-346, and 

 Vol. IV, pages -1 280- 1 284. 



t See Faraday's Experimental Researches, Series 19, 1846. A description of 

 Faraday's experiments and of later experiments along the same line is given in 

 Wiedemann, Die Lehre von der Elektricitdt, Vol. Ill, pages 907-968. 



J See Wiedemann, Die Lehre von der Elektricitdt, Vol. Ill, pages 192-194. 



See Wiedemann, Die Lehre von der Eltktricitdt, Vol. II, pages 126-136. 



