.1. M. Mayer — A larg\ Spring-balance Electrometer, 515 



ami when the apparatus is charged wo can then always depress 

 the mica-plate to the same level, and with the same force, by 

 dually raising the earth-plate. 



The helical spring is connected with the Leyden battery. 

 which latter is charged with 4- electricity from an electrical 

 machine. To insure a constant charge during the two meas- 

 ure* required to determine a specific inductive capacity, a 

 simple torsion balance carrying a gilded pith-ball is repelled by 

 i similar ball at the end of a rod connected with the battery. 

 A pointer on the torsion-rod moves over a graduated arc. 



To show the general phenomena of the action of a dielectric 

 compared with air, we charge the apparatus and bring up the 

 earth-plate till the mica-disc is sensibly depressed by the attrac- 

 tion existing between it and the plate connected with earth. 

 The disc of "the dielectric is now slid on the earth-plate, when 

 every one sees a considerable depression of the mica disc by 

 the increased force between the two plates through the inter- 

 vention of the dielectric. Or, we may reverse the operation, 

 and first place dielectric on earth-plate, then slowly raise this 

 plate, with dielectric, till the attraction between earth-plate 

 and disc has caused the latter to be depressed so far that it is 

 just above the point from which it would descend to contact 

 with the dielectric. 2s ow suddenly slide dielectric from the 

 earth-plate. The disc at once rises through a considerable dis- 

 tance, which is distinctly visible throughout the lecture-room. 

 In the case of a glass disc of 2 cms. thick this upward motion 

 may amount to 3 cms. and after a few oscillations the mica- 

 disc comes to rest at a distance of about \\ cms. above its 

 previous level. 



To show the " electric absorption," " residual charge," " per- 

 meability," or "dielectric afterworking " (Boltzmann) of a 

 dielectric we use a disc of paraffin. With this on the apparatus 

 we raise the earth-plate till the mica-disc is sensibly attracted, 

 and is stationary. This condition, however, only lasts for a 

 short time. Soon the disc begins to descend, and after a 

 minute or two has come near the paraffin, to whose surface it 

 now rushes. It cannot now be detached till the battery has 

 been discharged, or the earth-plate has been lowered so much 

 as to unduly stretch the spring. 



This experiment presents clearly a fact long known, that, 

 with the exception of sulphur, all dielectrics, and especially 

 paraffin are "permeated," or "polarized," in the condenser by 

 time-action of induction, and so the attraction existing between 

 the plates is increased. Boltzmann* shows from his experi- 

 ments that with times of inductive action lasting ■${-§ to ? ! ¥ 



* Wiener, SitziiDgsber., Ixviii, 1873. 



