348 Dr. E. B. Rosa on the Specific Inductive 



where C depends on the difference of potential between the 

 electrodes. / has a maximum value at a point P where 



A 



that is, about a quarter of the distance between the axes of 

 the cylinders from the origin. About this maximum point 

 the force varies but little for a considerable distance. This 

 is then a particularly favourable place to put the solid S, 

 as an exact knowledge of its position is not necessary. A 

 fairly accurate adjustment was, however, always made, by first 

 bringing the solid just between the electrodes where the 

 resultant force is zero, and then turning the torsion-head, by 

 means of a lever reaching to the telescope, always through 

 the same angle. The resultant force/ on the solid is inversely 

 proportional to A 3 , and directly proportional to the volume of 

 the cylinder, that is to the cube of its linear dimensions. Thus 

 changing the linear dimensions of the system does not alter the 

 force on the solid. The following is a brief statement of the 

 results observed : — 



1. Glass cylinders in air, petroleum oils, bisulphide of 

 carbon, turpentine, and benzene are attracted ; that is, 

 move into the stronger parts of the field. 



2. The force is least in air and greatest in turpentine; in 

 short is greater in those media having a greater specific in- 

 ductive capacity, all, however, being considerably less than 

 that of glass. This indicates that the principal change of 

 energy is in the fluid medium. 



3. In water, alcohol, and ether (the latter containing water 

 and alcohol as impurities) glass is repelled, the forces of 

 repulsion being nearly in the ratios of 9:3:1 respectively. 

 In bisulphide of carbon containing about 20 per cent, of the 

 same ether, glass is attracted, but with about 40 per cent, 

 it is repelled. 



4. The force of repulsion on glass in water is over 60 times 

 as great as the attraction in air. Since this is approximately 

 the value of the specific inductive capacity of water it suggests, 

 as in the case of glass in liquids of low specific inductive 

 capacities, that the chief change in energy due to the presence 

 of the glass is in the fluid media. 



5. The force on the glass is proportional to the square of 

 the difference of potential between the electrodes, and there- 

 fore to H 2 . In the case of water and alcohol this was verified 

 quite accurately ; and in the case of the other liquids, where 

 the forces were much smaller, it was verified approximately. 



