VOL. LXXII.J PHILOSOPHICAL TRANSACTIONS . 2(50. 



the other rod ; then it is evident, that the intensity of the electricity, by being 

 parted between the 1 rods, will be diminished in proportion as the capacity is in- 

 creased ; so that the index of the electrometer, which before was elevated to 6*0°, 

 will now fall to 10°, viz. to \ of the former intensity, because now the capacity 

 is 6 times greater than when the same quantity of electricity was confined to the 

 first rod alone. For the same reason, if the said quantity of electricity was to 

 be communicated to a rod 6o times longer, its intensity would be diminished to 

 1 degree : and, on the contrary, if the electricity of this long conductor was to 

 be contracted into the 6oth part of that capacity, its intensity would be increased 

 to 60°. 



Now not only conductors of different bulk have different capacities for holding 

 electricity, but also the capacity of the same conductor may be increased or 

 diminished by various circumstances, some of which have not yet been properly 

 considered. It has been observed, that the capacity of the same conductor is in- 

 creased or diminished in proportion as its surface is enlarged or contracted, as is 

 shown by Dr. Franklin's experiment of the can and chain, and various other ex- 

 periments ; from which it has been concluded, that the capacity of conductors is 

 in proportion to their surface, and not to their quantity of matter. This conclu- 

 sion is true, but does not comprehend the whole theory, since even the exten- 

 sion contributes to increase the capacity ; so that, of 1 conductors, which have 

 equal but dissimilar surfaces, that which is the more extended in length has the 

 greater capacity. In short, it appears from all the experiments hitherto made, 

 that the capacity of conductors is in proportion, not to the surfaces in general, 

 but to the surfaces which are free, or uninfluenced by an homologous 

 atmosphere. But that which comes nearer to our case is, that the capacity of a 

 conductor, which has neither its form nor surface altered, is increased when, in- 

 stead of remaining quite insulated, the conductor is presented to another con- 

 ductor not insulated ; and this increase is more conspicuous, according as the 

 surfaces of those conductors are larger and come nearer to each other. 



When an insulated conductor is opposed or presented to any other conductor 

 whatever, Mr. V. calls it a conjugate conductor. The circumstance mentioned 

 above, which augments prodigiously the natural capacity of conductors, is that 

 which has been hitherto principally overlooked : but let us (says Mr. V.) begin 

 with those experiments which show this increased capacity in the simplest manner. 

 I take, for example, the metal plate of an electrouhorus, and holding it by its 

 insulating handle in the air, electrify it so high that the index of an electrometer 

 annexed to it might be elevated to (30° ; then lowering this metal plate by de- 

 grees towards a table or other conducting plain surface, I observe that the index 

 of the electrometer falls gradually from 60° to 50°, 40°, 30°, &c. Notwith- 

 standing this appearance, the quantity of electricity in the plate remains the 



