THE ELECTRON 95 



theless have in it equal and unlike electricities. With 

 this in mind we now see how an atom or a molecule 

 may normally appear uncharged and yet have within 

 it equal and unlike electricities. 



Recognizing, then, that the atom is composed of a 

 nucleus of positive electricity and of electrons we may 

 now explain the electrification which is produced 

 by rubbing the glass with silk. During the intimate 

 contact some of the electrons which are not very 

 firmly held in their respective atoms become attached 

 to other atoms. During this rearrangement 1 of the 

 electrons the glass loses electrons to the silk. The silk 

 has more than its usual number. Conversely, the 

 glass rod has less than its normal number of electrons 

 and therefore appears positively charged. In other 

 words, for the glass rod there are not sufficient electrons 

 to neutralize in effect the positive nuclei of its atoms. 

 The positively charged glass rod will attract the neg- 

 ative silk with which it was rubbed, and it is against this 

 attraction that we do work in separating the two dis- 

 similar substances in the act of producing electrification. 



We may also ask why a charged glass rod attracts an 

 uncharged pith ball, for it was this phenomenon which 

 first called attention to electricity. The mechanism is 

 simple. The rod, being positive, draws toward itself 



1 We may imagine a very simple parallel case to illustrate how 

 this happens. Consider two trains which meet at a railroad siding. 

 Some of the men wander about from car to car, either of their own 

 train or of the other. But the men of train 1 stick more closely to 

 then* own cars than do those of train 2. When the whistle blows 

 for departure the first train will have more and the second train 

 less men than when they pulled in. If we picture the cars as 

 atoms and the men as electrons we have qualitatively the process 

 of "electrification by friction." 



