684 Prof. L. T. More on Dielectric 



accurately equi-distant. Alternate metal plates are then 

 connected by wires into two sets : one set to be charged 

 positively and the other negatively. The number of metal 

 plates must be odd, so that the top and bottom plates may be 

 charged with electricity of the like sign and connected together 

 rigidly by metal rods. Each plate in this system is thus 

 attracted equally up and down, or is electrically in equilibrium. 

 Whereas, if the dielectric itself expands or contracts along the 

 lines of force, the whole pile will tend to shorten or lengthen 



according to the formula ii-y = - — -^. where d is the thick- 

 et oiTcr 



ness of each dielectric plate, K the dielectric constant, V the 



potential, n the number of dielectric plates in the pile, and jju 



is Young's modulus. 



To accomplish this object, it was necessary to cast a large 

 number of plates, which should be exactly parallel, equi- 

 distant and close together, in a dielectric, to form a solid block. 

 A great deal of difficulty was found in getting a dielectric 

 which was a good insulator for high potentials, and at the 

 same time rigid and not viscous, and which would not contract 

 when solidifying. The last was very important and debarred 

 paraffin, as this and similar substances contract sufficiently 

 to fissure in the thin places between the electrodes. The 

 substance finally adopted w T as a mixture of four parts by 

 weight of the best shellac, one of resin and two of Venice 

 turpentine. This compound gives a clean and smooth surface 

 when broken, and is highly elastic (Young's modulus 

 about 10 10 ). It melts at 110° and when solidified shows little 

 contraction, — a two-litre flask, half full of the hot liquid, 

 presents an almost level surface when it solidifies. 



Several types of moulds for the casting were made and 

 tried before the one was devised which answered. Thirty- 

 one brass plates, 5 cm. in diam. and 2*5 mm. thick, were 

 polished and made true. Three lugs, 12 mm. long and 6 mm. 

 wide, were soldered radially to each plate. A seamless brass 

 tube, 21*5 cm. long and 7*4 cm. inner diameter, was then 

 ruled with thirty-one circular lines, each 5*5 mm. apart, and 

 three holes were bored and tapped, equally distant apart on 

 each line. The plates were then placed each in the plane of 

 one of these circles, with a lug opposite each hole and held 

 in position by three small pointed screws for each plate. 

 Two of the plates are shown mounted in a section of the tube 

 in fig. 1. In the next to the top plate was screwed a brass 

 tooth of the same dimensions as the ebonite tooth shown in 

 fig. 2 (p. 686), and a hole was bored in the top plate so that 

 the tooth passed through it. The bottom plate was tapped 

 with three screw-holes and a large plate, only a little less in 



