MAGNETISM AND ELECTRICITY 195 



rubber, and we say the rubber rod has a negative charge. 

 We simply mean that some of the electrons which belong 

 on the cloth have been removed to the rod and that for 

 the present the atoms of the rubber rod are overcrowded 

 with electrons. Now, since the electrons are little par- 

 ticles of negative electricity, of course the rod will ex- 

 hibit a negative charge. We must remember that the 

 little electrons have like charges and repel each other ; 

 so whenever we have this overcrowding of electrons, we 

 find them pushing each other in all directions as if to 

 make their condition less crowded. In fact, if any other 

 object is brought near, a number of them will be pushed 

 over on it. As the number of these electrons on a given 

 insulated area is increased, the crowding becomes so great 

 that finally they are forced across great air gaps with a 

 disruptive charge ; as is the case when an electrical 

 machine is operated or as in the case of lightning. 



If glass is rubbed with silk, we say the glass receives a 

 positive charge, but what really happens is that the elec- 

 trons let go of the glass rod and collect on the silk. There 

 is a lack of negative particles on the glass, and we say it 

 has a positive charge. 



An unelectrified body is composed of atoms in which 

 the strength of the charge on the positive nucleus is 

 exactly counterbalanced by the sum of the negative 

 charges or electrons. 



Charging a Body by Induction. Suspend two egg- 

 shells which have been covered with tin foil by silk 

 threads, so that they touch each other (Figure 164). Bring 

 a positively charged rod near the shell A. While the 

 rod is in this position both shells will be electrified. A 

 will have a negative charge and B a positive charge; 

 the reason being that some of the electrons (negative 



