CONDENSATION OF GALVANIC ELECTRICITY, 649' 



and covered with a thin insulating layer of shellac. 

 The lower plate is supported by an insulating pillar, 

 to the upper an insulating handle is attached. The 

 apparatus is therefore very similar to that in fig. 322, 

 but that the single projecting insulating plate, which 

 is rather thick, is replaced by two very thin insulating 

 layers which need not project beyond the plate, because 

 the tension is in this case so small that a passage of the 

 spark across the edges of the intervening insulator 

 cannot take place. 



Metal plates cannot well be made quite flat and smooth without 

 a turning- lathe and some skill in the work ; but we may use plates 

 of stout window-glass, each 8 cm in diameter, and cover them with 

 tinfoil. The plates are prepared exactly like the adhesion plates 

 (see page 155) ; they need not be polished with jeweller's rouge, 

 but some care should be bestowed upon the grinding of the edges, 

 which must be very smooth. A pillar and handle of sealing-wax are 

 fixed as in the condenser, fig. 322 ; they must be attached before 

 covering the plates, or the tinfoil would come oft' when the plate is 

 heated to the melting point of sealing-wax. 



To avoid breaking the sealing-wax whilp'the plates are being 

 covered with tinfoil, they should be placed upon s.ome support which 

 has a suitable aperture, for example a filter-holder or a wide-necked 

 bottle. Upon each plate a disc of tinfoil is laid, having its diameter 

 20 or 25 mm larger than that of the plate. Before placing the tin- 

 foil upon the plate, in order to, ensure a perfectly smooth contact 

 between them, attention should be paid that no particles of dust 

 adhere to either plate or tinfoil, and that no hard granules are in 

 the layer of paste. The tinfoil is now pressed gently upon the 

 plate with the finger, beginning in the middle and drawing the 

 finger in all directions towards the edge. As soon as the middle 

 portion is pretty firmly attached, the tinfoil is bent over the edge 

 upon the back of the plate, using only moderate pressure. When 

 the edge of the tinfoil is thus turned over all round the plate, and 

 attached to the back, the operation of pressing the tinfoil every- 

 where close to the plate must be repeated until the paste has been 

 squeezed out from between glass and metal as thoroughly as possible. 

 Gradually greater and greater pressure will be required for this 

 purpose, and in leading the finger from the middle, it is essential that 



