Oscillatory Discharges. 15 



the distribution of the discharge over the different parts of the 

 circuit, it was first of all necessary that the condenser should 

 not offer such difficulties as would arise from the penetration 

 of the charge and from a delay of polarization, which always 

 occur with condensers having solid dielectrics. 



This could be obtained only by adopting an air-condenser 

 constructed especially for this investigation. 



It was made up of 70 plates of mirror-glass, plane, coated 

 with tinfoil on both sides and separated from one another by 

 small glass prisms. 



The mirror-glass plates are rectangular ; their surface is 

 70 x 35 cms., with a thickness variable from one plate to 

 another, and ranging from 7 to 10 mm. On each of them, 

 as has been mentioned, there is extended on both faces a thin 

 sheet of tinfoil of one piece, caused to adhere to the glass by 

 special precautions, so as to prevent folds as well as any air- 

 bubbles beino- formed. The tinfoils, after being extended 

 on the glass, were all cut to the exact dimensions 63 x 28 cms., 

 so as to leave free around them a margin of glass 3*5 cms. in 

 breadth. The two tinfoils of each plate were connected to 

 each other by means of a thin brass strip (about 4 mm. in 

 breadth) , which at the same time served to establish convenient 

 communications. 



These 70 plates were arranged in two piles of 35 each,the first 

 and last plates of either pile having tinfoil on the internal face 

 only. As the interval between two successive plates should 

 remain unaltered and well known, each couple was separated 

 by six small glass prisms, chosen equal among themselves to 

 a hundredth of a millimetre by means of a spherometer. 



The prisms of the pile of condensers No. 1 we found to be 

 of a mean thickness of 0*743 cm., those of the pile No. 2 

 being of a mean thickness of 0*738 cm. In both of these two 

 condensers the even-numbered and the uneven-numbered 

 plates respectively were put in connexion with each other, 

 the respective brass strips being then gathered in two clusters, 

 which were connected to two terminals carried by glass rods. 



Each condenser was placed on a solid wooden bench, 

 the respective plates being carried laterally by six glass 

 angles, so as to secure absolute stability. The apparatus was 

 finally protected by a glass jacket, in the interior of which the 

 air was kept dry by means of sulphuric acid. 



(b) Measurements of Capacity. 

 14. As the dimensions of our condenser are known with 

 accuracy, its capacity could be derived from the well-known 

 formula of Maxwell (-treatise/ vol. i. § 196). 



