RECENT PROGRESS IN PHYSICS. 



391 



35. 



eleventh series of Experimental researches in Electricity^ (Pog, An. 

 XL VI, 1) lie endeavors to prove that induction is not the consequence of 

 electrical action at a distance, but is effected by the inducing Ijody 

 through the medium of the intervening material particles. 



In order to prove that induction is the result of an action progress- 

 ing from particle to particle of the separating insulator, Faraday seeks 

 to prove — 



1st. That at the same distance of the inducing body and that on 

 ■which electricity is excited and rendered latent by induction, the force 

 of the induction is dependent upon the nature of the intervening insu- 

 lator ; that the induction under otherwise like conditions is not the 

 same through different insulators ; that therefore to each insulator 

 belongs a peculiar specific inductive capacity. 



2d. That induction can take place in curved lines. 



§ 22. Specific inductive capacity. — We will first consider the spe- 

 cific inductive capacity of insulators. 



In fig. 35, A represents a hollow metallic 

 sphere, standing on a metallic support. In an 

 opening at the top a cylinder of shellac is fast- 

 ened tightly, through the middle of which a 

 wire passes, having at its upper end a small 

 metallic knob K, aud at the lower the metallic 

 ball B. The diameter of the sphere A is about 

 8.5 centimetres, (3^ inches,) that of the ball B 

 is 6 centimetres, (2|^ inches). The sphere A 

 consists of two pieces similar to the Magdeburg 

 hemispheres, and so arranged that the upper 

 half can be removed together with the shellac 

 cylinder and, the balls K and B. 



Faraday, in his experiments, used two per- 

 fectly similar instruments of this kind, which he 

 termed an inductive apparatus. 



Such an apparatus can be charged like a 

 Leyden jar^ by bringing K into contact with a 

 source of electricity, and connecting A with the 

 ground. Thus, B represents the inner coat- 

 ing, A the outer, and the stratum of air between 

 takes the place of the glass. 



An apparatus of this kind, which I shall indicate by I, was charged 

 as above shown. It is evident, as in the case of the inner coating of 

 a Leyden jar, that there mu3# be an excess of free electricity on B and 

 K, the tension of which was measured by Coulomb's torsion balance. 

 In order to maintain the centre of the two balls of the balance at an 

 angular distance of 30°, a torsion of the thread of 250° was necessary. 



The knob K of the apparatus I •was then touched by the knob K of 

 a perfectly similar apparatus II, while its exterior sphere was in con- 

 nexion with the earth. The charge, which had been previously com- 

 municated to apparatus I alone, was now divided between the two. 

 After this division, the force of the free electricity of the interior was 

 determined for each ; the first corresponded to a torsion of 124°, the 

 other to a torsion of 122° of the balance, in order to maintain the balls 



