2 ELECTRICAL ENGINEERING 



In the first class are silver, which is the best conductor, copper 

 and other metals, graphite, impure water and solutions of salts. 

 In the second class are air, which when dry is an almost perfect 

 insulator, glass, paraffin, ebonite, porcelain, rubber, shellac, oils 

 and the numerous insulating compounds used in electrical engi- 

 neering. 



3. Electrostatics and Electromagnetics. Electrostatics com- 

 prises phenomena related to electric charges at rest and to the 

 stresses produced in the field surrounding them. These phe- 

 nomena become of great importance where very large differences 

 of potential must be provided for, as for example in the design 

 and operation of all high voltage apparatus and systems. 



Electromagnetics comprises phenomena related to electricity 

 in motion, that is, to currents of electricity and the magnetic 

 fields produced by them. Almost all the problems to be solved 

 by the electrical engineer come under this head. 



4. Laws of Electrostatics. First Law. Like charges of elec- 

 tricity repel one another; unlike charges attract one another. 



Second Law. The force exerted between two charges of elec- 

 tricity is proportional to the product of their strengths and is 

 inversely proportional to the square of the distance between 

 them; it also depends on the nature of the medium separating 

 them. 



This law can be expressed by the formula, 



/- ........ w 



where q and qi are the charges of electricity, r is the distance 

 between them in centimeters, K is a constant depending on the 

 medium separating the charges and is called its specific inductive 

 capacity or dielectric constant. The unit of quantity is so chosen 

 that the dielectric constant for air is unity; for all other sub- 

 stances it is greater than unity. / is the force in dynes exerted 

 between the two charges; if the charges are of the same kind the 

 force between them is a repulsion and / is positive. 



One electrostatic unit of quantity is that quantity which, when 

 placed at a distance of one centimeter in air from a similar quan- 

 tity, repels it with a force of one dyne. 



The practical unit of quantity is the coulomb; one coulomb 

 is 3 X 10 9 electrostatic units. 



