CH. XIII] DIRECT CURRENT UNITS 477 



657. The Ohm. This is the unit of resistance to the flow of 

 an electric current. It is represented by the resistance, at zero 

 centigrade, of a column of mercury 106.3 centimeters long, of uni- 

 form cross sectional area, and weighing 14.4521 grams. Such a 

 column of mercury will have a cross sectional area of one square 

 mm. 



Ohmage is a term analogous with voltage and amperage. It is 

 used to designate the amount of resistance in ohms of an electric 

 circuit. 



A conductor may have little resistance, as copper, etc., or it may 

 have great resistance like German silver. Naturally then copper 

 wire is used largely for electric circuits, and German silver wire for 

 making resistors or rheostats ( 7240.}. 



658. The Watt. This is the unit of activity and is the rate 

 at which work can be done by a current of one ampere under a 

 pressure of one volt. One watt means the doing of work at the 

 rate of io 7 ergs per second, or one joule per second. This is approx- 

 imately equal to the lifting of i kilogram, io centimeters every 

 second. 



659. Kilowatt. A kilowatt is 1,000 watts. This term is 

 more common than watt. It is equal to 1.34 horse power. 



660. The watts which any direct current represents are 



obtained by multiplying the quantity of current flowing by the 

 pressure that is, the amperes by the volts. Thus, if there were 

 an amperage of one and a voltage of one, there would be an 

 activity of one watt. If the voltage were io and the amperage 100, 

 or the voltage 100 and the amperage io, there would be an activity 

 of 1,000 watts, or one kilowatt. 



661. Kilowatt-hour. This is the unit of electrical energy or 

 work, which is in commercial use and which is used as a basis for 

 making the charges to consumers. A kilowatt-hour is the work 

 represented by one kilowatt when acting for one hour. 



In order to find the amount of work done by an electric current 

 it is necessary not only to know the rate at which the work is being 

 done but also the time during which this rate is continued. Thus, 



