THE PHENOMENA OF ELECTROSTATICS. 



209 



Fig. 137. 



hurst machine consisting of two coaxial glass cylinders rotating 

 in opposite directions. The negative charges on both cylinders 

 are collected by the double metal comb on the left as the rotating 

 cylinders pass between the prongs of the comb, and the positive 

 charges of both cylinders are collected by 

 the double comb on the right. 



116. Electroscopes. An electroscope is 

 a device for indicating the existence of an 

 electric charge, or for detecting an electric 

 field. 



The pith ball electroscope consists of a 

 gilded ball of pith suspended by a silk thread. 

 The presence of an electric field in a given 

 region may be shown by charging the pith ball, and noting the 

 force which acts upon it when it is placed in the given region, 

 the direction of the field being indicated by the direction of the 

 force which acts upon the ball. 



A pith ball may be hung alongside of a body of metal, as 

 shown in Fig. 137. If the body of metal is charged, a portion 

 of the charge is given to the ball, and the lines of force which 

 emanate from the ball pull it outwards from the body as shown 

 in the figure. 



The essential features of the gold leaf electroscope are shown in 

 Fig. 138. A metal rod R is supported in the top of a glass case 

 cc by means of an insulating plug, a metal disk D is fixed to 

 the upper end of the rod, and two gold leaves are hung side by 

 side from the lower end of the rod. The glass case cc serves to 

 protect the gold leaves from air currents. The sides of cc are 

 lined with strips of metal foil ff, and these pieces of metal should 

 be connected to earth. When the disk, rod and leaves are charged, 

 the leaves are pulled apart by the lines of force which emanate 

 from the leaves and terminate on the strips ff as shown in Fig. 

 139. This figure shows the instrument without the enclosing 

 case for the sake of clearness. 

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