CH. XIII] ELECTRIC CURRENTS 475 



653. Alternating current. This is characterized by flowing 

 first in one direction and then in the opposite direction ; the polarity 

 is therefore constantly changing. (See 676). 



Alternating current is pro- 

 duced only by dynamos. It is 

 used especially for the trans- 

 mission of power to great dis- 

 tances, incandescent lighting, 

 arc lighting, for motors and for 



FIG. 252. CONNECTIONS OF A th electric furnace, as in the 

 VOLTMETER TO MEASURE THE 



LINE VOLTAGE. manufacture of carborundum 



G Dynamo. and graphite. 



A Arc Lamp. Alternating current has the 



R Rheostat. advantage of being more easily 



Note that the terminals of the volt- pro duced, as the dynamo is Sim- 

 meter are connected to the two points * . 

 between which it is desired to measure pier; but its great superiority 

 the potential difference. In this case lies in the f act that practically 

 it is the main supply (across the line). 



without loss it can be stepped 



up or down in voltage by stationary transformers. This makes it 

 possible to raise it to a very high voltage (1000 to 100,000 volts) 

 for transmission to a distance over wires of moderate size. It is 

 then stepped down in voltage before it is used. In this process of 

 stepping up or down in voltage the amperage takes the reverse 

 direction, so that the product of the volts by the amperes is a 

 constant quantity. 



The disadvantages of alternating current for the arc lamp are: 



1 . The arc is not as bright as with the same amperage of direct 

 current. 



2. The light is intermittent. 



3. The alternating current arc is noisy. 



ELECTRIC UNITS AND THEIR MEASUREMENT 



654. Electric Units. For the purposes of this book it is 

 necessary to refer frequently to electric units, like the volt, the 

 ampere, the ohm and the watt; it seems proper therefore to give 

 a brief discussion of these units. 



