﻿M. W. von Bezold's Investigations on the Electrophorus. 223 



the two positions upon the cover must be as D to d* ; D being* the 

 thickness of the cake, and d the thickness of the layer of air be- 

 tween it and the form (relatively to the cover) . But this ratio 



-j is ^in any case a very large number. If, however, the cake 



is removed from the form, the positive electricity collected upon 

 the form loses its influence, and to a greater extent the higher 

 the supports ; the primary again begins to operate, and the cover 

 must therefore yield positive electricity. 



Third Experiment. — If while the cake lies on the form it be 

 pretty strongly rubbed, after dusting the rubbed surface there 

 will in general be no essential difference from the previous case. 

 Only when the rubbing is too strong, which may be recognized 

 by a crackling sound when the cover is laid upon the cake, 

 can a circle of yellow rays and stars be observed in those parts 

 over which was the edge of the cover. We will first of all neg- 

 lect this case, and assume that the upper surface presents the 

 appearance previously described; it will now, on the contrary, 

 be found that the lower surface exhibits a highly remarkable 

 and for the most part very beautiful image, and one which dif- 

 fers according to whether the cake during the dusting (with the 

 surface A) was on the form or on high supports. 



In the former case the entire surface is covered with yellow 

 stars which have partially a red central spot ; in the latter case 

 no sulphur sticks to the surface, but the same stars appear 

 black (that is, free from dust) upon the surface. If the dusting 

 has been effected in the first position, and if then the cake be 

 raised, the sulphur flies from the stars towards the edge. 



This experiment teaches that during the rubbing the space 

 between the form and the cake is traversed by sparks, and the 

 positive electricity strikes on the cake in the form of the well- 

 known stars. 



But the quantity of this positive electricity is far smaller than 

 that of the negative which is on the surface A, because, when the 

 latter is not bound by the positive electricity attracted in the 

 form, the action of the primarily excited electricity preponderates ; 

 for after the raising the sulphur is not attracted, but repelled, by 

 those parts, which are undoubtedly covered with positive elec- 

 tricity. 



This affords an excellent example how the electricity present 

 at a definite place may be made electroscopically imperceptible 

 by the stronger action at a distance of quantities elsewhere dis- 

 tributed. 



The accuracy of the opinion above expressed may be tested 

 by a further experiment ; for if it is true that the positive elec- 

 * More accurately as D+d to d. 



