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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 produced only by dynamos. It is used especially for the transmission of power to great distances, 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 produced, as the dynamo is Simmeter 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 fact 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.