in Electric Induction. 405 



eoiaes between the metallic discs, a diminution which will be dis- 

 regarded in the following experiments; this experiment alone, 

 however, is necessarily at variance with the assumption, that 

 the induction of a charged metallic disc is stronger through 

 shell-lac than through air, inasmuch as no reason can be assigned 

 why a small piece of shell-lac should weaken an action which a 

 greater piece strengthens. The experiments described at the 

 end of the present memoir will furnish the explanation of this 

 remarkable ph?enomenon. 



In order to operate with smaller intermediate plates, the large 

 metallic discs were replaced by smaller ones, 4"45 inches in dia- 

 meter, at a distance of 1*65 inch from one another. Here, how- 

 ever, an important influence on the result of the experiment was 

 produced by changing the position of the clamp. On this account 

 the clamp was abandoned, and above the metallic discs a metallic 

 cylinder 3 inches long was fixed, in whose axis a metallic rod 

 could turn softly ; this rod carried at one end a round nut for 

 adjustment, and at the other a clamp li inch long. With 

 respect to this clamp, a convenient arrangement may be here 

 noticed ; one of its legs is immoveable, and the other can be 

 moved on a prism by means of a screw, so as to keep the two 

 legs always parallel to one another. The handle of a plate being 

 fixed in this clamp, a rotation of 90° given to the axis of the 

 cylinder was sufficient either to bring the plate between the 

 metallic discs, or to remove it sufficiently from them, without 

 altering the position of the apparatus towards the discs, exception 

 being made of the screw on the clamp, whose influence, when 

 present, was taken into account. This arrangement of the ap- 

 paratus enabled the observer to preserve his arm and hand un- 

 moved during an experiment, inasmuch as a slight pressure of 

 the finger was sufficient to bring the plate into the two required 

 positions. The immoveability of the observer is necessary in some 

 of the following experiments, in which importance is attached to 

 small motions of the gold leaves on the electroscope. In the 

 experiments immediately following, the indications of the elec- 

 troscope were so decided as to render this precaution unneces- 

 sary. The following table gives the changes which, according 

 to the two-fold indications of the electroscope, were produced in 

 the induced electricity of the neutral metallic disc by interpo- 

 sing the several plates there mentioned. The interposed plates 

 were fixed to glass or shell-lac pillars, from 4 to inches long, 

 and before the experiments were completely deprived of their 

 electricity. 



