TABLES 513-521.— MAGNETO-OPTIC EFFECTS 503 



Faraday discovered that, when a piece of heavy glass is placed in magnetic 

 field and a beam of plane polarized light passed through it in a direction parallel 

 to the lines of magnetic force, the plane of polarization of the beam is rotated. 

 This was subsequently found to be the case with a large number of substances, 

 but the amount of the rotation was found to depend on the kind of matter and 

 its physical condition, and on the strength of the magnetic field and the wave- 

 length of the polarized light. Verdet's experiments agree fairly well with the 

 formula 



= clH 



(-4;) 



where c is a constant depending on the substance used, / the length of the path 

 through the substance, H the intensity of the component of the magnetic field 

 in the direction of the path of the beam, r the index of refraction, and A the 

 wavelength of the light in air. If H be different, at different parts of the path, 

 IH is to be taken as the integral of the variation of magnetic potential between 

 the two ends of the medium. Calling this difference of potential v, we may 

 write 6 = Av, where A is constant for the same substance, kept under the 

 same physical conditions, when the one kind of light is used. The constant A 

 has been called "Verdet's constant," and a number of values of it are given in 

 Tables 514-517. For variation with temperature the following formula is 

 given by Bichat : 



R = R (1 - 0.00104* - 0.000014/ 2 ), 



which has been used to reduce some of the results given in the table to the 

 temperature corresponding to a given measured density. For change of wave- 

 length the following approximate formula, given by Verdet and Becquerel, 

 may be used : 



fli _ Mi 2 (/*i 2 ~ 1)A 2 2 

 6 2 (i2 2 (i*2 2 — l)Ai 2 



where (x is index of refraction and A wavelength of light. 



A large number of measurements of what has been called molecular rotation 

 have been made, particularly for organic substances. These numbers are not 

 given in the table, but numbers proportional to molecular rotation may be de- 

 rived from Verdet's constant by multiplying in the ratio of the molecular weight 

 to the density. The densities and chemical formulas are given in the table. In 

 the case of solutions, it has been usual to assume that the total rotation is simply 

 the algebraic sum of the rotations which would be given by the solvent and dis- 

 solved substance, or substances, separately ; and hence that determinations of 

 the rotary power of the solvent medium and of the solution enable the rotary 

 power of the dissolved substance to be calculated. Experiments by Quincke 

 and others do not support this view, as very different results are obtained from 

 different degrees of saturation and from different solvent media. No results 

 thus calculated have been given in the table, but the qualitative result, as to the 

 sign of the rotation produced by a salt, may be inferred from the table. For 

 example, if a solution of a salt in water gives Verdet's constant less than 0.0130 

 at 20°C, Verdet's constant for the salt is negative. 



As a basis for calculation, Verdet's constant for carbon disulfide and the 

 sodium line D has been taken as 0.0130 at 20°C. 



SMITHSONIAN PHYSICAL TABLES 



