1835.] Philosophical Transactions for 1834. 



231 



that the electricity passed in preference from the heated side rather 

 than from the cool side. To make this experiment free from objec- 

 tion, it would be necessary to insert the heated iron rod in an 

 exhausted receiver. Dr. Ritchie was aware of this, but conceives 

 that the effect of a heated wire would be a species of electrical eva- 

 poration from its surface. His very ingenious paper in the philoso- 

 phical transactions has certainly not attracted that attention which it 

 deserves. The objection stated to his experiment by Mr. Harris, 

 does not appear to affect the results which he obtained. 



8. Volta observed that of two plane surfaces of equal area, that 

 which has the greatest extension has also the greatest capacity for 

 electricity. Mr. Harris has prosecuted this fact, and ascertained that 

 the intensity varies in an inverse ratio of the perimeter of plates 

 which he employed, varying in shape from a circle through a square 

 up to a long parallelogram. The following illustrates the results : — 



DIMENSIONS, — AREA = 75 SQUARE INCHES. 



The extent of edge has no influence on the intensity. The inten- 

 sities of conductors are therefore, it appears, inversely as their peri- 

 meters, and the intensity varies in an inverse ratio of the area when 

 the perimeters remain the same, from which, it follows that the 

 intensity must vary inversely with those quantities jointly, or calling 

 I, intensity, A, area, P, perimeter, we have 



IO AP 



But supposing the quantity of electricity to vary, then the intensity 

 being as the square of the quantitv, the formula is 



t * 2 

 ^ AP 

 and the capacity of a conductor being measured by the quantity of 

 electricity it can receive under a given intensity, there follows *2 a 

 I A P, or with a constant intensity, * representing the capacity, we 



obtain capacity . 



- V AP 



It appears that the intensity does not vary in an inverse ratio of 

 the square of the surface according to the general law, except when 

 the areas are so disposed that the whole perimeter of the various 

 plates is as the respective surfaces. 



9. The operation of electricity on distant bodies, by induction, is 

 quite independent of atmospheric pressure, and is exactly the same in 

 vacuo as in air, the attractive force varying as the squares of the 

 respective distances inversely. 



1st. The attractive force exerted between an electrified and a 

 neutral uninsulated conductor, is not at all influenced by the form 

 or disposition of tjie unopposed portions. 



2d. The force is as the number of attracting points in operation 



