208 Royal Society. 



were varied, the force varied as the squares of the distances between 



the centres of force, or according to the formula Foe , .^y y 



With the view of further verifying these results, a set of plane 

 circular plates in pairs, each pair equal in area to the areas of the 

 respective hemispheres of the spheres, were submitted to experiment 

 at the same constant distance Tl, so as to cut the points qc/, or 

 centres of force of the spheres ; the attractive forces were found pre- 

 cisely the same as that of the opposed spheres to which the particular 

 plates belonged. 



The author has examined at various times and with very rigid at- 

 tention, the several conditions under which electrical attractive force 

 conforms to the law of force as deduced by Cavendish and Coulomb, 

 and other eminent philosophers, and he finds this law true only for 

 charged and neutral conductors of large inductive capacity ; if either 

 of the attracting surfaces have a narrow or limited susceptibility of 

 inductive charge, then this law of force no longer obtains. If, for 

 example, the attracting plates be taken as mere planes of small thick- 

 ness, and even although they be charged with opposite electricity, 

 still in changing the distances between them we do not obtain at all 

 distances a law of force in the inverse duplicate ratio of the distance 

 such as we have found to obtain in other circumstances. The force 

 will be commonly in an inverse simple ratio of the distance. If the 

 neutral or suspended plane be taken very thin and insulated, then 

 little or no attractive force is observable under any circumstances. 

 If we continue to increase its thickness, then, as the author has 

 shown in former papers*, attractive force begins to display itself, 

 and will approach a law of change in the inverse duplicate ratio of 

 the distance as we extend its dimensions. When we give it unli- 

 mited electrical extension by placing it in communication with the 

 ground, then the force is as the square of the distance inversely; but 

 it is not always so, until we effect this extension perfectly. When 

 all these sources of disturbance are duly considered, it will not be 

 difficult to reconcile the many conflicting results arrived at by several 

 eminent philosophers in past times, in their endeavours to investigate 

 the law of electrical force, and explain how, without any defect in 

 their experimental processes, such conflicting results might arise. 

 Tolta, for example, found electrical force to vary in a simple inverse 

 ratio of the distance. M. Simon, of Berlin, an eminent philosopher, 

 and eulogized by Gilbert as being " remarkable for his dexterity and 

 careful manipulation" in this branch of physics, failed to verify 

 Coidomb's result, although he employed a new and very delicate 

 apparatus, by which the repulsive force between two spheres was 

 very accurately and beautifully measured. In these experiments he 

 found the force to vary as the distance inversely f. 



* Phil. Trans, for 1834. 



t Poggend. jinnal. for 1808, cap. 3, p. 277, ami Ann. de Chim. vol. Ixix. First 

 Series. 



