158 SCIENTIFIC APPARATUS. 



point. In Coulomb's torsion electrometer, the instrument by 

 which that philosopher (1785) established the fundamental laws 

 of electric force, as well as in Dellmann's, Peltier's, and 

 Kohlrausch's electrometers, the indications are proportional to 

 the difference of potential between the body under examina- 

 tion and the earth. In the various electrometers devised by 

 Sir William Thomson, especially in the Quadrant Electrometer, 

 whether as constructed by him or as modified and simplified 

 by Branly, the comparison is usually between two insulated 

 conductors. 



Electrical Accumulators and Condensers. 



Insulated conductors are needed for the accumulation of elec- 

 tricity. When a conductor, originally unelectrified, is charged 

 with electricity, its electrical potential, or degree of electrification 

 (positive or negative) rises in direct proportion to the quantity of 

 electricity that is given to it. The consequence of this is that the 

 magnitude of the charge which can be imparted to a given 

 conductor is limited in two different ways. In the first place, 

 there is always a practical (if not a theoretical) limit to the 

 electrical potential which can be produced by the electrical 

 machine, or other source of electricity, that is employed to 

 charge the conductor; and when the conductor has received 

 sufficient electricity to make its potential the same as that of the 

 source, it cannot receive any more. Secondly, as the potential of 

 the conductor rises, its tendency to impart electricity to other 

 bodies increases, and hence, if more and more electricity is con- 

 tinually supplied it, the leakage consequent on the imperfectly 

 insulating character of the supports, or due to discharge through 

 the surrounding air, becomes after a time equal to the supply, and 

 then the charge cannot any longer increase. The quantity of 

 electricity which an insulated conductor can receive without 

 having its potential raised beyond a given limit, depends partly 

 on the extent and form of its surface, and partly on the position 



