200 



FRICTIONAL ELECTRICITY. 



remember that attraction affords no safe test for the kind of elec- 

 tricity, while repulsion does. If glass rubbed with silk repels a body, 

 that body is charged with + electricity. If sealing-wax rubbed 

 with flannel repels a body, that body is charged with electricity. 



331. Electroscopes. An instrument used to 

 detect the pres- 

 ence of electric- 

 ity, or to deter- 

 mine Us ~kind, is 

 called an electro- 

 scope. The elec- 

 tric pendulum 

 ( 324) is a com- 

 mon form of the 

 electroscope. Two 

 vertical strips of 

 tissue paper, hang- 

 ing side by side, 

 constitute a simple 

 electroscope. It is well to prepare the paper beforehand 

 by soaking in a strong solution of salt in water and drying. 

 The gold leaf electroscope is represented in Fig. 139. A 

 metallic rod, which passes through the cork of a glass ves- 

 sel, terminates below in two narrow strips of gold leaf and 

 above in a metallic knob or plate. The object of the 

 vessel is to protect the leaves from mechanical disturb- 

 ance by air currents. The upper part of the glass is often 

 coated with a solution of sealing-wax or shellac in alcohol, 

 to lessen the deposition of aqueous vapor from the atmos- 

 phere. This instrument when well made is very delicate. 



FIG. 139. 



332. Use Of the Electroscope. (a.) The electric pendu 

 lura is used as an electroscope as follows : If the uncharged pit!* 



