GENERAL ACCOUNT OF COMMON 



which can bo closed by the cork c when the instrument is not in 

 use, in order to prevent the entrance of dust. p,p are two brass 

 side plates which can be arranged at any distance on each side of 

 the gold leaves, the sensitiveness of the instrument being greater 

 the nearer the plates are to the leaves. If the wood of the case is 

 non-conducting the side plates should be connected by wire to 

 earth. The reason for this will be seen when we have considered 

 the phenomena of induction. The top of the rod may conveniently 



end in a metal table /. If this is touched by an electrified body 

 some of the electrification is communicated through it to the rod 

 and leaves, and the leaves being similarly electrified, repel each 

 other and sta:id out as in the figure. 



An important improvement in the construction of the 

 gold-leaf electroscope which is now usual consists in prolonging 

 the central rod by a thin metal strip, and having only one 

 gold leaf which is attached to the strip near its upper end. The 

 fixed vertical strip then takes the place of the second gold leaf. 

 On electrification the gold leaf stands out from the strip. Fig. 4 

 represent* a form of the instrument suitable for lantern projection. 

 It i> convenient to have a glass scale engraved on one of the glass 

 sides. 



Electrification by induction. We have described above 

 the charging of the electroscope by contact with an electrified 

 body. Hut contact is not necessary. On bringing an electrified 



