Mr. C. V. Boys on Quartz as an Insulator. 



15 



centre of the lid of a mahogany box Fig. 1. 



lined with tinfoil. From this is 



suspended the piece of glass or 



quartz, B, bent to the form shown, 



so that it can be handled by the 



projecting end without touching 



the portion that acts as the insu- 

 lator. On this hangs a piece of 



bent brass, C, to which the leaves, 



D, are attached. A small ring of 



wire is soldered to the upper end of 



C, which is used when one insulator 



is changed for another as follows : — 



A stiff wire, E, passes through one 



side of the instrument, and this is 



pushed forward through the wire 



ring. A is then depressed until the 



lower side of B hangs clear of the 

 hook C. E is then drawn back 

 with C and D suspended from it. 

 Finally, when B is changed, E is 

 pushed forward again, and A raised 

 until the ring on is just free 

 from E, which is then withdrawn. 

 By this means the leaves are always 

 left at the same level. The leaves are 90 x 5 millim., and 

 the box, which has a glass front and back, is 285 millim. 

 high x 130 millim. wide, and 185 millim. from front to back. 

 The leaves are suspended so that the line of junction of C and 

 D is 160 millim. above the base of the instrument. The 

 length of the insulating portion of the quartz and glass hooks 

 B is 21 millim. and the diameter about 1 millim. The leaves 

 were observed by fixing the object-glass of a telescope at a 

 distance of 393 millim., which projected an image of the 

 leaves on a scale 1940 millim. beyond the lens. Thus the 

 observed divergences were 4*93 times the true distances 

 between the ends of the leaves. 



The results of the experiments can be seen from fig. 2 

 (Plate I.). The rate at which the leaves close is the same with 

 lead-glass in air dried by sulphuric acid, with quartz in air 

 dried by sulphuric acid, and with quartz in air kept moist by 

 means of a large flat dish of water. Soda-glass in air dried 

 by sulphuric acid allowed the electricity to escape about 

 eleven times as fast. With either kind of glass in moist air 

 the charge escaped almost at once, but soda-glass was much 

 worse than lead-glass. The glass had in all cases been boiled 



