80 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



consequently, because of its insulated condition, be positive externally, as at its back 

 or at the gold leaves ; whilst C would be less negative towards A, and therefore ne- 

 gative outwards or at the gold leaves. This was found to be the case ; for on which- 

 ever side of A the shell-lac was introduced the external plate at that side was positive, 

 and the external plate on the other side negative towards each other, and also to un- 

 insulated external bodies. 



1310. On employing a plate of sulphur instead of shell-lac, the same results were, 

 obtained ; consistent with the conclusions drawn regarding the high specific induc- 

 tive capacity of that body already given (12/6.). 



1311. These effects of specific inductive capacity can be exalted in various ways, 

 and it is this capability which makes the great value of the apparatus. Thus I intro- 

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

 uninsulated them, and finally left them in the insulated state ; the gold leaves were 

 of course hanging parallel to each other. On removing the shell-lac the gold leaves 

 attracted 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 difference 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 enlarging 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 resulted. So that in fact the 

 gold leaves became, in this manner, 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, per- 

 haps, bring out differences which have as yet escaped me (1292. 1293.). 



1314. It is also evident that two metal plates are quite sufficient to form the in- 

 strument ; the state of the single inducteous 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 inducteous surfaces may even be balls ; the latter being itself the carrier ball of 

 the Coulomb's electrometer (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 for 



