276 PHYSICS. 



Kerr has extended his electro-optical researches, and by the aid of new 

 and improved apparatus has confirmed his early conclusions. The ap- 

 paratus includes a paral!iu-oil lamp ; a ISTicol with its principal section 

 incUned 45° to the horizon ; a cell for the liquid so arranged that the 

 line of junction of the electrodes is horizontal ; a system of compensat- 

 ing glass plates which can be submitted to a vertical traction by means 

 of known weights ; a neutralizing plate of glass of irregular structure, 

 to compensate tlie double refraction of the cell ; a hand-comi)ensator ; 

 a vertical strip of glass which can be flexed by the hand parallel to the 

 axis of the luminous bundle, designed to determine the direction of the 

 double refraction produced in the liquid ; and an analyzing Nicol in- 

 clined at 45° to the horizon and 90° to the principal section of the x)olar- 

 izer. When adjusted the light is completely extinguished and can be 

 restored by the hand-compensator. The electric machine is then put 

 into action. With carbon disulphide a fraction of a turn suffices to re- 

 turn the light, which instantly disappears again on discharging the con- 

 ductor. The compensator shows a uniaxial double refraction like that 

 of glass submitted to traction parallel to the lines of force. With nitro- 

 benzene the light reappears only on drawing a spark from the second 

 electrode. In a subsequent paper, Kerr formally enunciates the fol- 

 lowing law : "The intensity of electro-optic action of a given dielectric 

 (or the difference of retardations of the ordinary and extraordinary rays) 

 per unit of thickness of the dielectric varies directly as the square of 

 the resultant electric force." On the question of stress he says : "The 

 dio[)tric action of an electrically charged medium is closely related to 

 the electric stress of the medium, the axis of double refraction coincid- 

 ing in every case with the line of electric tension and the double refrac- 

 tion varying, certainly in OS2, and probably in all other dielectrics 

 directly and simply as the intensity of the tension." — {Phil. Mag. V., viii, 

 85, 1879; ix, 157, 1880 ; J. Plujs., viii, 414; ix, 255, 1880.) 



ELECTRICITY. 



1. Magnetism. 



Carre has introduced into Paris the manufacture of magnets of su- 

 perior quality made of cast iron. His process is as follows : A soft and 

 very slightly carburetted metal is melted in earthen crucibles, and just 

 before running it into the moulds 10 or 15 per cent, of steel filings are 

 added. In order to produce a metal which will stand tempering at a 

 cherry-red heat, there is added either 1 to 1.5 per cent, of nickel, with 

 0.25 per cent, of copper, or 2 per cent, of tin and 0.5 per cent, of copper. 

 —{Nature, xxi, 359, February, 1880.) 



Obalski has described to the Academy a very neat magnetic experi- 

 ment. Two magnetic needles are hung vertically by fine threads, their 

 unhke poles being opposite one another. Below them is a vessel con- 

 taining water, its surface not quite touching the needles, which are 



