516 A. M. Mayer — A large Spring-halance Electrometer. 



sec. ; t 9 -q sec. ; 1 T 8 ¥ sec. ; and 45 sees., the specific inductive 

 capacity of paraffin is respectively 2*32 ; 2"51 ; 2*56 ; and 8*12. 

 Schiller's* experiments show that for times of charging plate- 

 glass in the condenser of Yjrbn "o an( ^ 2V sec -> tne specific induc- 

 tive capacity is respectively 5 83 and 6*34, or an increase of 

 9 per cent in "03995 of a second. Wullnerf gives these con- 

 clusions from his experiments on the subject: "The instan- 

 taneous capacity (that is, the capacity when the charging lasts 

 for a very small fraction of a second) is of quite a different 

 nature to that capacity which increases slowly as the electrifica- 

 tion continues. The 'instantaneous capacity' is independent 

 of the conductivity — the 'slow capacity' is not." 



To measure the specific inductive capacity of a substance, 

 we place the guard-ring around the mica- disc. This ring has 

 projecting from the edge of its circular opening three short fine 

 wires, so adjusted that when the mica-disc rests on these wires 

 the lower surfaces of disc and guard-ring are in the same plane. 

 The apparatus having been charged, the dielectric is intro- 

 duced and the earth-plate is raised till the mica-disc is attracted 

 so that it rests on the three wires of guard-ring. The plate is 

 now very slowly lowered till the tension of the spring just ex- 

 ceeds .the attraction of plates, when the disc suddenly rises. 

 The scale reading on support of earth-plate or of micrometer- 

 screw is read. The dielectric is now removed and the disc 

 brought down on the wires by raising earth-plate and then by 

 lowering this plate the disc again leaves the wires, and the cor- 

 responding scale reading is taken. From the difference of the 

 scale readings the specific inductive capacity is determined, as 

 follows : 



Let the scale reading when only air is between the plates be 

 s\ when the dielectric is introduced, s / ; then 8,—s is the dis- 

 tance the earth-plate has been moved so as to give in the two 

 experiments the same force of attraction on the mica-disc, A 

 dielectric of thickness t and specific inductive capacity K equals 

 in capacity a condenser with a thickness of air between plates 



equal to — . Now when we introduce the dielectric plate it 



replaces a thickness of air, t. So the effect of introducing the 

 dielectric is to increase the electric capacity of the apparatus 

 as if we had brought the earth -plate and the mica-disc nearer 



by this distance t and then farther apart by the distance — , 



which is the same as if we had diminished the distance between 



* Pogg. Ann., clii, 1874. 



f Sitzungsb. kongl. bayer. Acad., 1877. 



