36 Nature of the differential Inductometer. 



1311. These effects of specific inductive capacity can be 

 exalted in various ways, and it is this capability whicli makes 

 the great value of the apparatus. Thus I introduced the 

 shell-lac between A and B, and then for a moment connected 

 B and C, uninsulated them, and finally left them in the insu- 

 lated state ; the gold-leaves were of course hanging parallel 

 to each other. On removing the sliell-lac the gold-leaves at- 

 tracted each other; on introducing the shell-lac between A 

 and C this attraction was increased, (as had been anticipated 

 from theory,) and the leaves came together, though not more 

 than four inches long, and hanging three inches apart. 



1312. By simply bringing the gold-leaves nearer to each 

 other I was able to show the diflFerence of specific inductive 

 capacity when only thin plates of shell-lac were used, the rest 

 of the dielectric space being filled with air. By bringing B 

 and C nearer to A another great increase of sensibility was 

 made. By enlarghig the size of the plates still further power 

 was gained. By diminishing the extent of the wires, &c. 

 connected with the gold-leaves, another improvement re- 

 sulted. So that in fact the gold-leaves became, in this man- 

 ner, as delicate a test of specific inductive action as they are, 

 in Bennet's and Singer's electrometers, of ordinary electrical 

 charge. 



1313. It is evident that by making the three plates the 

 sides of cells, with proper precautions as regards insulation, 

 &c., this apparatus may be used in the examination of gases, 

 with far more effect than the former apparatus (1187. 1290.), 

 and may, perhaps, bring out differences which have as yet 

 escaped"^me (1292. 1293.). 



131 4'. It is also evident that two metal plates are quite suf- 

 ficient to form the instrument; the state of the single induc- 

 teous plate when the dielectric is changed, being examined 

 either by bringing a body excited in a known manner towards 

 its gold-leaves, or, what I think will be better, employing a 

 carrier ball in place of the leaf, and examining that ball by 

 the Coulomb electrometer (1180.). The inductive and in- 

 ducteous surfaces may even be balls ; the latter being itself 

 the carrier ball of the Coulomb electi'ometer (1181. 1229.). 



1315. To increase the effect, a small condenser may be 

 used with great advantage. Thus if, when two inducteous 

 plates are used, a little condenser were put in the place of the 

 gold-leaves, I have no doubt the three principal plates might 

 be reduced to an inch or even half an inch in diameter. Even 

 the gold-leaves act to each other tor the time as the plates of 

 a condenser. If only two plates were used, by the proper 



