ELECTRICAL ENGINEERING 



CHAPTER I 



ELECTROSTATICS 



1. Electrification. Bodies which are charged with electricity 

 are said to be electrified. Charges are of two kinds called posi- 

 tive and negative. Bodies which have a positive charge are 

 acted upon by forces tending to make them give up their charge; 

 bodies which have a negative charge are acted upon by forces 

 tending to convey a positive charge to them equal to their nega- 

 tive charge. These forces are exerted through the medium 

 separating the charges and the medium is in a state of stress. 



The body with the positive charge is at a higher potential 

 than the body with the negative charge and the difference of 

 potential between the two is a measure of the tendency for elec- 

 tricity to pass from one to the other. 



2. Electrical Conductors and Insulators. If two metallic 

 bodies charged to different potentials are joined by a metal wire, 

 electricity will flow from one to the other until the potential of 

 both is the same and the transfer of electricity will take place 

 almost instantaneously. The metal wire is therefore a good con- 

 ductor of electricity; or, it offers a low resistance to the passage 

 of electricity through it. 



If the two charged bodies had been joined by a glass rod, there 

 would have been no transfer of electricity between them, or, it 

 would have taken place so slowly that it could only be detected 

 by the most delicate instruments. Glass is therefore a very bad 

 conductor; or, it offers a very high resistance to the passage of 

 electricity. It is called a non-conductor or insulator. 



As all materials conduct to a certain extent, it is not possible 

 to divide them absolutely into conductors and insulators, but, 

 since the resistance of a good insulator is many million times 

 that of a good conductor, they may be so divided for practical 

 purposes. 



