ELECTRICAL MACHINERY 467 



will flow very much faster from the short pipe than from 

 the long one. From this we learn that the pressure de- 

 creases as the water passes through the pipes and the 

 longer the pipe the more it falls. The friction between 

 the water and the inside of the pipe retards the flow 

 and is known as resistance. Electricity flowing over a 

 wire is an analogous case. The current meets with re- 

 sistance in the wire and there is a fall in potential. 



Comparative resistance. To measure comparative re- 

 sistance, silver is the unit of comparison, it having the 

 lowest resistance of any substance. 

 Specific resistance of some metals : 



Silver, i.oo; 



Copper, 1.13; 



Aluminum, 2.00; 



Soft iron, 7.40; 



Hard steel, 21.00; 



Mercury, 62.70. 



Laws of resistance. As the lengths of wire increase the 

 resistance increases and as the diameter increases the re- 

 sistance decreases. Hence the following law is deduced : 

 That the resistance of conductors of the same materials 

 varies in direct proportion to length and inversely to the 

 area of the cross-sections. 



The resistance of iron increases with rising tempera- 

 ture, likewise with nearly all metals, while the resistance 

 of carbon and liquids decreases as the temperature in- 

 creases. 



Unit of resistance. A conductor maintaining a P. D. of 

 one volt between its terminals and carrying a current of 

 one ampere is said to have a resistance of one ohm. The 

 ohm is the unit of resistance and is named in honor of 

 George Ohm, the great German physicist. 

 Ohm's law. The current existing in a circuit is always 



