4 STATIC ELECTRICITY 



stage between moderate insulation, such as that of wool or that of 

 baked wood, and good conduction, such as that of the metals. 

 Sometimes a body may be in itself an insulator, but may have a 

 tendency to condense water from the air on its surface, and t 

 of water will form a conductor. This explains the difficulty 

 experienced in keeping electrification on glass or the still greater 

 difficulty in keeping it on paper. Sometimes the surface of tin- 

 insulator decomposes. This often happens with vulcanite and 

 glass, and the products of decomposition form a conducting layer. 

 The insulation of glass may be improved by immersion for a short 

 time in boiling water. Dust may also prevent insulation by form- 

 ing a continuous coating of conducting matter on the surface of a 



ng 

 body. 



yrkivliich .slitlc.9 un 

 down, the rcci. 



t'ilnT j 

 . \ulfilttirn 



The hygroscopic tendency of glass greatly impairs its efficiency 

 as an insulator except in a very dry atmosphere, It is. then-fore. 

 necessary in electrical experiments with glass either to replace the 



glass surface by another less 

 hygroscopic or to take special 

 precautions to keep the surface 

 dry. Often the g88 Ifl co\ n<l 

 with shellac varnish or with 

 sulphur. A very simple plan to 



preserve the insulating property 

 of a vertical glas.s rod is to allow 

 it to end in a lead foot fitting 

 tightly at the- bottom < 

 narrow glass jar (Fig. X). .\bo\c 

 the lead is placed some asl H -sto> 

 fihre moistened \\ith strong sul- 

 phuric- acid. Sliding on the rod 

 is a cork which may be brought down to close the jar when not in 

 use. The sulphuric acid dries the nir of the- jar thoroughly, and 

 the glass is therefore also dry. When in use the cork is slightly 

 raised so that there is no connection between the- upper and 

 possibly damp part of the rod and the jar except through the rod 

 itself, which is insulating. 



Gold-leaf electroscope. The two electrified discs //. I) 

 (Fig. 1) will serve as indicators of charge on another body if that 

 charge is considerable. Fora charged body will repel one or other 

 of the discs. But a much more delicate test of the existence of 

 electrification is given by the gold-leaf electroscope, which in its 

 original form consists essentially of two strips of gold leaf hanging 

 from the lower end of an insulated metal rod in n glass case, the 

 upper end of the rod projecting above the case. Fig. 3 is a 

 common type of instrument. g,g are two long, narrow strips of 

 gold leaf hanging from the end of the brass rod ?;-, which is fixed 

 in nn insulating bar of vulcanite rv stretching across the case. 

 The rod passes freely through a large hole in the top of the c 



