50.2 M. Verdet on the Optical Properties developed in 



magnetic action at different distances from the terminal surface 

 of this armature*, and observed a very slow decrease, which may 

 fitly be represented by a decreasing geometrical progression, 

 although it may be in reality a little less rapid. 

 This may be seen in the following Table : — 



Ratio of each magnetic 

 action to the preceding. 



0-76 

 0-78 

 0-80 

 0-81 

 0^83^ 



Mean . . . (F796 



I repeated the experiment after an interval of two months, 

 during which time the electro-magnet had been very frequently 

 put in use, and I obtained again the same law, as the table shows. 



Mean . . . 0-785 



Thus, by the effect of time and of use, the electro-magnet did 

 not appear to be modified, so that the law of its action on an ex- 

 ternal point was sensibly changed. We may then be permitted 

 to believe, that when M. Bertin made his experiments in 1847 

 and 1848, if he had measured the magnetic actions exercised at 

 different distances, he would have obtained results perfectly 

 similar to the preceding ; he would therefore have been able to 

 represent these magnetic actions by a decreasing geometric pro- 



* This method only gives the value of the magnetic action exactly when 

 this action is constant in the space where the inductive bohbin moves ; but 

 if the values of the magnetic action at different points of this space are 

 slightly different, it is clear that the methods determine practically the mean 

 value. 



t The magnetic actions inserted in this column have been determined 

 by the method of alternatives, in order to eliminate the influence of small 

 variations of the intensity of the current. The pile was composed of 10 

 Bunsen's elements. 



+ See preceding note. 



