MAGNETISM AND ELECTRICITY 



397 



Quantitative measurement is more difficult, but Prof. 

 Mayer has lately used a pendulum electroscope in a form that 

 makes a most sensitive electro- 

 meter. He suspends a gilt 

 pith-ball I cm. radius (built up 

 of small pieces cemented) a 

 distance of 364 cm. from the 

 ceiling, by two silk fibres 

 fastened 52 cm. apart and 

 meeting at the ball, behind 

 which a scale is arranged. A 

 brass ball of the same size is 

 mounted on a glass rod, var- 

 nished while warm with 

 paraffin wax. A force of only 

 one dyne acting on the sus- 

 pended ball deflects it 13-3 

 mm. ; and charging both balls 

 in contact so as to give the 

 same charge to each, the 

 charge c on either was found 

 in absolute electrostatic units 

 by the formula 



where D is the distance in 



cm. between the centres of 



the balls, and d the deflection 



from the vertical in cm. 



When D is over 5 cm. the 



law of inverse squares was 



demonstrated within 1 per Via. siffaX^ m i/<*' ^ 



cent; and by using a proof- /^^^/ ^ c^ 



plane, Coulomb's law of distribution on the pajp^J? a/^iin-<^ 



drical body was verified with close approximj 



