Z ELEMENTS OF STATIC ELECTRICITY. 



attractive force, though variable in intensity, is com- 

 mon to all. 



The balanced rod, represented in Fig. 1, will be 

 found more convenient for these experiments than the 

 balanced spoon. It consists of a round wooden rod, 

 about twenty inches long, and half an inch in diameter, 

 with the ends rounded or terminating in balls. It is 

 pivoted at the center on a point, and may be mounted 

 on a stand, or y on a bottle with a pin through the cork, 

 and made to revolve rapidly by the force of attrac- 

 tion, following any of the electrified bodies already 

 mentioned w r hen held near it, as represented. 



Fig. 1 The Balanced Rod. 



A more sensitiveinstrument for investigations of this 

 class is represented in Fig. 2, and known as the pith- 

 ball electroscope; the name electroscope being derived 

 from the Greek cxoTteco, to see, ifaxrQov,. electricity. It 

 is constructed as follows : A small brass rod, bent at 

 right angles, has its short arm inserted into an ebonite 

 stem attached to a wooden base, giving the instrument 

 a vertical height of about Hi inches. The horizontal 

 arm is about 8 inches long, and terminates 'in a small 

 brass ball. From this arm two pith balls, each about 

 half an inch in diameter, are suspended by silk threads. 



