13 



near the brass there is likewise no attraction (Fig. 10), but when the ebonite rod 

 is charged and held near, but not touching the brass body, we find that the pith 

 ball is attracted! (Fig. 11.) 



Thus we see the brass body has been charged not by either friction or contact, 

 but by the influence of the charge on the efbonite rod a little distance away. The 

 brass is said to have been charged by "induction." 



A Condenser Increases the Charge. 



Two brass plates on glass stands' are so arranged that they can be placed face 

 to face. Each plate has a small pith ball attached by a cotton thread, which will 

 show when the plate is charged. From the ebonite one of these plates is now 

 charged by touching it a number of times with the rod which is each time rubbed 

 with the catskin. The pith ball is strongly repelled, and stands out from the plate 

 almost in a horizontal position. (Fig. 12.) 



Fig. 13. — The same plate as in figure 12 

 (with the same charge) after plates were 

 placed close together, but not touching. 

 Note that the pith-ball has dropped show- 

 ing less repulsion. More electricity can 

 now be given to the charged plate. That 

 is why the plates when close together 

 are called a " condenser." 



Fig. 14. — The same condenser as in 

 Figure 13. Note pith^ball horizontal 

 again as in Figure 12. As much ex- 

 tra charge was added to the left-hand 

 plate as the plate originally held. The 

 right hand plate has become charged 

 by " induction," as shown by repulsion 

 of pitb-ball. 



Now when the other plate is brought close to the charged one we note that the 

 pith ball drops considerably (Fig. 13), and more electricity can, be put on the 

 charged plate, in the same manner as before, until the ball again stands in a 

 horizontal position. (Fig. 14.) 



Two insulated plates, side by side, are, consequently, called a "condenser." A 

 condenser enables us to store up a much larger charge of electricity than either 

 plate alone will hold. 



Charges Reside on the Surface. 



One other law should be mentioned, viz. : When a conductor is electrified the 

 charge resides entirely on the surface. This can be readily proven, but the experi- 

 ment does not lend itself to illustration by photograph. 



