M. A. Bertin on Circular Magnetic Polarization. 499 



would be twice the action of a single one, whilst it is much 

 stronger. In one experiment, for example, it was 28° lO' in 

 the first case, and only 12° 30' when one of the coils was re- 

 moved. 



To sum up, we may state that the rotation produced by the 

 two coils of our apparatus is represented by the formula 



—ci^-^Yr^ + r^-"-"), 



which gives the action of a single coil in making d= co . 



In this formula r appears neither to depend on the intensity 

 of the magnetism nor on the nature of the substance. As to 

 c, it depends on both ; but it remains constant in all the ex- 

 periments compared, because they were always made at very 

 short intervals, and moreover upon the same substance, and 

 with the same distance between the poles. 



It would undoubtedly be curious to ascertain why c varies 

 with the intensity of the magnetism ; but it may already be 

 said that the law is the same for all substances ; so that the 

 relations of the rotations produced by these substances do not 

 depend on the force of the magnetism, as may be seen by the 

 following experiment made with M. Becquerel's electro- 

 magnet. 



Flint-glass of Mr. Faraday, Sulphuret of carbon, Relation between 

 of 18'3 millims. of 10 millims. the rotations. 



7 42 3 18 0-43 



13 48 6 00 0-43 



19 00 8 18 0-43 



Three other experiments made with M. RuhmkorfTs ap- 

 paratus gave — 



For Faraday's flint-glass 

 For Matthiessen's flint-glass 

 For common flint-glass . . 



And in these three series the rotation of the flint-glass of 

 M. Matthiessen remained nearly equal to 0*8, and that of the 

 common flint-glass equal to 0*5 of the rotation produced by 

 the flint-glass of Prof. Faraday. 



For this reason I propose to call c the coefficient of magnetic 

 polarization. The value is calculated by comparing two ro- 

 tations observed at short intervals upon two substances placed 

 under certain circumstances, but always between two poles at 

 the same distance, that is to say, by deducing the value of c 



2 K 2 



