HEAT AND LIGHT FROM ELECTRIC CURRENTS 209 



small motor an ammeter and voltmeter combined for the 

 purpose of measuring the energy of the electric currents. 



In section 195 there was a discussion of the units of work. 

 That is, 



Work done = force (pounds) x distance (feet), 



and the unit of work is the foot-pound. The unit of power 

 is 550 foot-pounds per second, which is known as one horse 

 power. It must be understood that the unit of power is 

 an arbitrarily determined unit, but as necessary as are other 

 units of measurement. 



Units of work and power in the metric system may be 

 obtained in the same manner. 



Work done = force (grams) X distance (centimeters). 



The unit of work is the gram centimeter, and the unit of 

 power is 10,200 gram centimeters per second, or 1 watt. 



The watt is a rather small unit of power ; therefore in prac- 

 tice the kilowatt (1000 watts) is used. One horse power is 

 equivalent to 746 watts. The watt was named in honor of 

 James Watt (1736-1819), who invented the steam engine. 



The number of watts used in a direct-current circuit 

 can be obtained by multiplying the amperes by the volts. 

 For example, the power consumed by the electric toaster 

 (fig. 100) is calculated as follows : 



Watts = amperes x volts 

 Watts = 5 x 110 

 Watts = 550 

 550 watts = .55 kilowatt 



If the cost of electricity is ten cents per kilowatt for an 

 hour, the cost of operating the toaster will be five and one-half 

 cents per hour, eleven cents for two hours, etc. The cost of 

 energy in an alternating-current circuit, which is more com- 

 monly used, is obtained by means of the watt-hour meter in- 

 stead of by the method described above. Watt-hour meters are 

 made for both direct-current and alternating-current circuits. 



