ACTION OF A DIELECTRIC SPHERE. 481 



This method was used by Boltzmann.* 



In the first series of experiments the ratio of the forces X and X 

 was determined by deflections from the vertical. Two small spheres 

 of sulphur 07 cm. in diameter, one of which is coated with gold 

 leaf to make it a conductor, are suspended at a distance of 

 9 cm. from two wires 2 metres in length placed in front of a 

 divided scale. When a metal ball 2 '6 cm. in diameter, originally 

 electrified, is inserted between them, the two balls are attracted 

 unequally. If the experiment be so arranged that the active ball is 

 in the centre of the interval of the movable balls, the ratio of the 

 attractions is equal to the ratio of the deflections, which is observed 

 by the microscope, or, more generally, by the product of the ratio 



Fig. 228. 



of the deflections by the ratio of the weight of the balls. The 

 method is not very accurate, because it is difficult to insure equality 

 of distance and to avoid the action of currents of air. 



1071. Another arrangement employed Uy Professor Boltzmann 

 is more delicate. The movable ball is attached by two wires to the 

 end of a metal rod (Fig. 228), itself supported by a bifilar sus- 

 pension F, so as to form a small torsion balance. A mirror M 

 serves to measure the deflections. The needle of this balance is 

 protected by a screen against air currents. The active ball com- 

 municates with a Leyden jar, to which a definite charge has been 

 imparted by a spark electrometer (824). By means of a conducting 

 torsion balance connected with the earth, and analogous to the first, 



* BOLTZMANN. Wiener Sitz. Berichte, Vol. LXVIII., Pt. ii., p. 81. 1873. 

 VOL. II. I I 



