CH. XIII] ELECTRIC MEASUREMENTS 43 



If the wattmeter were wrongly connected, the instrument could 

 not register the watts on the one hand and on the other it might be 

 injured. 



670. Portable wattmeter. This has a pointer which shows 

 directly in watts, the power consumed at a given instant. 



671. Stationary or house 

 wattmeter. The wattmeter for 

 the electric supply looks some- 

 thing like a gas meter for the gas 

 supply. It is of the integrat- 



FIG. 256. WATTMETER TO MEASURE in g ^P 6 - is permanently con- 



THE POWER DELIVERED BY THE nected with the line, and con- 



DYNAMO. taing & wheel the ed of w h OS e 



G Dynamo. . , 



W Wattmeter. rotation is directly proportional 



A Arc lamp. to fa e pow er consumed. This 



R Rheostat. 



The fine wire connects the watt- wheel turns pointers over the 

 meter with the line where the power is dials on which are indicated 



kilowatt-hours. The numbers 



toward which the pointers are directed indicate the kilowatt- 

 hours which have been used, just as the pointers in a gas meter 

 indicate the number of cubic feet of gas which have been used. 

 For example, by consulting the wattmeter before and after an 

 exhibition one can see how much work, measured in kilowatt-hours 

 has been consumed by the arc light. 



Suppose the voltage of the line were no, and the voltage between 

 the carbons is 55 volts. Suppose the amperage is 20, then the 

 watts should be (volts times amperes) 55x20 = noo watts at any 

 instant, and for an hour, for example, it would be 1 100 watt hours, 

 or i.ioo kilowatt-hours. 



67 la. With both direct and alternating current, when a rheostat is in 

 the circuit, the amperage may be found by the aid of the stationary wattmeter, 

 this is always present in a house supply of electricity, as is the gas meter for the 

 gas supply, and one does not always possess an ammeter. 



It is necessary to know the voltage of the line. This is usually 1 10 or 220. 



One must also know the watts, or kilowatts at any given instant. This can 

 be found by the wattmeter as follows: Suppose the reading is 1.87 kilowatt- 

 hours. As this number was obtained by multiplying volts x amperes x time, 

 and the time is one hour, then the kilowatts of power consumed is 1.87. The 



