138 ELECTRICITY 



charge in a will cause a corresponding change in the 

 charge of b. But since a induces a negative charge in 

 the nearer portion of 6, any change that makes the 

 potential of a more positive will result in charging that 

 portion of b still more negatively, that is, even more 

 unlike the charge in a. 



Thus we see that a difference in potential ( 156) 

 exists between an inducing charge and the nearer por- 

 tion of the charge that it induces ; also that as the 

 inducing charge becomes more intense this difference 

 becomes greater. Now difference in potential gives rise 

 to electro-motive force, which can overcome resistance; 

 so when the difference between the potentials of the 

 inducing charge and the induced charge is great enough 

 to overcome the resistance of the insulator that separates 

 these two charges, a spark passes between them. The 

 path of this spark through air is a good conductor at 

 that instant, and electricity passes from one charge to 

 the other, making them of equal potential. This sudden 

 and rapid flow of electricity is called a discharge. The 

 sparks seen when a cat's fur is rubbed are caused by 

 discharges between the fur and the hand. 



161. Lightning. Lightning is an electric discharge 

 similar to that just explained, but on a far greater 

 scale. The positively and negatively charged bodies 

 are generally clouds, though in many cases the nega- 

 tive charge is on the earth. In some instances the 

 positive charge is on the earth, the negative charge 

 being in the clouds. The clouds are more commonly 

 charged during the very rapid and heavy condensation 



