112 



Mr WJiiddington, The Shadow Electroscope 



these lines using an insulated brass sphere of radius .3-25 cm.*; 

 Fj and V^ are the scale readings in volts. 



From the above readings the mean value of Fj/Fg = 1'450, 

 whence C^ = 7-2 cm. 



Experiment 2. To determine the capacity of a parallel plate air 

 condenser by the method of Experiment 1 . 



The readings tabulated below were obtained with a specially 

 designed circular plate air condenserf . The diameter of the central 

 plate being 4-25 cm., and its distance from two outer earthed 

 plates being 0-15 cm., the capacity C'a can be calculated from the 



formula for a parallel plate air condenser, viz. 2 — -^ cm. 



Inserting the proper values for the present case leads to the 

 value 60-2 cm. 



The experimentally determined value may be expected, if 

 anything, to be rather greater than this calculated value owing 

 to the extra capacity of the edges of the central plate. 



The method is essentially the same as in Exp. 1 but in this 

 case the formula is 



vjv. 



1 



Using the same sphere as in Exp. 1 the following results were 

 obtained, the insulated central stalk of the condenser fitting in 

 the electroscope cup (see figure) and the outer plates being con- 

 nected to earth. 



* An ordinary bedstead knob mounted on an insulating ebonite rod. 



t The main point in the design is the protection of the central insulated plate 

 from dust, small hairs etc., which under the comparatively high potentials em- 

 ployed would be attracted to it with resulting insulation troubles. The central 

 plate is therefore sandwiched between two outside parallel plates, one of which is 

 provided with a peripheral spacing ring which in butting up against the other 

 outer plate completely encloses the inner insulated one. Insulating grooved buttons 

 of ebonite form the insulation. 



