32 Mr. O. J. Lodge. Experiments on [June 4. 



under which they are measured. Nevertheless, in order to get a 

 fairly good measure of their capacity, a standard condenser was made, 

 with which they could be compared. 



Standard Air-condenser. 



A couple of plates of carefully selected plate glass, as used for 

 mirrors, about 2 feet square, were silvered chemically on both faces, 

 secure connexion between the two faces round the edge of the top 

 plate being made. A circular cut or clean scratch, 1 mm. broad and 

 53*04 cm. inside diameter, was then made on the under surface of the 

 top plate so as to isolate a trap-door portion. A hole previously 

 drilled through the centre of the top plate, and silvered inside, per- 

 mitted conductive access to the central area ; and the borders of the 

 plate acted as guard ring. The silver was cleared away from a small 

 patch near the centre of the upper surface of the top plate, and a glass 

 tube cemented on permitted the trap-door terminal to emerge in a 

 well-insulated manner. Two other terminals, one attached to the 

 bottom plate, and the other to the general surface of the top plate 

 (the guard-ring terminal), were provided. 



Four glass distance-pieces with wide bevelled edges (to improve in- 

 sulation) were carefully cut out of one piece of glass, and their thick- 

 ness was measured with a spherometer as 0*12083 inch. 



The top plate was supported on these, near its four corners, and the 

 whole placed in a suitable box with artificially dried atmosphere, the 

 bottom plate being similarly supported near its corners, so that what- 

 ever bending there was might result in concentric surfaces. The 

 electrostatic capacity of the trap- door was thus 572*9 cm. 



There was some difficulty with the insulation of the trap-door por- 

 tion, and the main leak was traced to dust, viz., fine fibres of some 

 length, which settled on the bottom plate and bridged the interval 

 between it and the top one. The narrow gap between trap-door and 

 guard-ring, being of course carefully cleaned, was not found to leak 

 anything like so much as one of these fibres. By care, the causes of 

 leak could be minimised, and a special key was constructed whereby 

 the trap-door could be discharged through a ballistic galvanometer 

 the merest instant before the guard-ring was discharged to earth. In 

 fact, by a screw adjustment the two events could be made simul- 

 taneous. 



34. A set of 144 secondary Plante cells, made by bending strips of 

 lead over small glass vessels standing in a sort of test-tube rack, 

 having been charged in twelve sets of twelve each, were connected 

 in series and used to fill the condensers with. 



Sometimes different kicks were obtained with the whole series of 

 cells ; sometimes the same kick was imitated by tapping off a certain 



