Electricity and Light. 447 



For convenience of description, I shall suppose the block already 

 fixed across the two insulating pillars of glass, its long edges 

 horizontal, and plate-faces vertical. The pillars are in this case 

 about 6 inches apart, close to the ends of the block ; they pro- 

 ject also 2 or 3 inches above it. 



A narrow tunnel, straight and clean, is worked through the 

 block by drill and file at right angles to the plate-faces. The 

 tunnel is concentric with the block \ its two sides are sensibly 

 plane and perpendicular to the long edges of the block; its floor 

 and roof are slightly rounded. The length of the tunnel, or 

 thickness of the plate, is | of an inch, its width (horizontal) § of 

 an inch, and its height (vertical) about J of an inch, so that there 

 is a good margin of polished plate-surface all round each mouth. 

 This tunnel, with its mouths properly closed, forms the cell ; it is 

 usually charged with liquid through a fine vertical boring which 

 leads into it at the centre of the roof. 



The fine borings for the induction-wires lie here, as they did 

 in the dielectric of glass, midway between the plate-faces: but 

 they are not now horizontal ; they pass from the two upper cor- 

 ners of the block obliquely downwards, and enter the tunnel at 

 the centres of its opposite sides. 



The induction-wires are of copper, and about ^ of an inch 

 thick. Each ware is bent at about an inch from the end, and 

 closely doubled upon itself, so as to present a very well-rounded 

 extremity. This end of the wire enters from the boring a little 

 way into the tunnel. At the outer end of the boring each wire 

 is bent closely upon the block, and tied very firmly along 

 several inches of its length to the upper part of the adjacent 

 pillar. The parts of the two wires within the tunnel are easily 

 bent by pincers into proper form and position (virtually fixed), 

 so as to project from the sides of the tunnel towards its centre, 

 in one line parallel to the long edges of the block, and to about 

 g of an inch from each other. To an eye looking straight 

 through the tunnel, the induction-wires appear single, with per- 

 fectly rounded ends, exact, images of each other, lying in one 

 horizontal line, and projecting equally far from the sides of the 

 tunnel. 



Two small panes of plate glass, each about 2 inches square 

 and yg- of an inch thick, and of the best quality obtainable in 

 the glass-shops, are placed on opposite faces of the block, so as 

 to close the mouths of the tunnel with a good margin of contact 

 between plate- surfaces all round. Outside of each plate is a 

 square of india-rubber cloth of the same size. Outside of the 

 cloths are two stiff planks, each 2 inches (horizontal) by 4 (ver- 

 tical), which are connected above and below by nail and nut, so 

 as to form a weak screw-press. The cloths and planks are per- 



