484 ALTERNATING CURRENT UNITS [Cn. XIII 



UNITS AND THEIR MEASUREMENT WITH ALTERNATING CURRENT 



With alternating current there is, strictly speaking, no such 

 thing as voltage and amperage as the electric potential is varying 

 from instant to instant. Consequently a kind of average value of 

 the electric pressure and amount of current is used instead. 



672. Alternating current voltage. When alternating current 

 is measured, the voltage indicated on the voltmeter is the mean 

 effective voltage. 



In order that this average effective value for a volt shall corres- 

 pond as nearly as possible to the analogous value with direct cur- 

 rent, the value taken is the square root of the average of the squares 

 of the instantaneous values of the potential difference during an 

 entire cycle. Or briefly, it is the root mean squares of the instan- 

 taneous pressure. 



673. Alternating current amperage. The number of amperes 

 indicated on an ammeter when using alternating current represents 

 the mean effective amperage. The average effective value of the 

 ampere is, as with the volt, the square root of the average of the 

 squares of the instantaneous values of the current during an entire 

 cycle. 



voltage with a rheostat is the line voltage. Now as the kilowatts are the pro- 

 duct of voltage by amperage divided by i ,000 and both the voltage and the 

 kilowatts are known the amperage can be found by multiplying the kilowatts 

 by 1,000 to reduce them to watts, and dividing the watts by the voltage = 1 10. 

 1870 -H 1 10 = 17 amps. With alternating current, if an inductor (choke-coil) 

 is used for regulating the current, the wattmeter can also be utilized for deter- 

 mining the amperage at the arc, for by experiment it is known that no matter 

 what the line voltage is, the voltage across the arc is usually about 34 volts. 

 The fall of potential across the inductor does not count. The wattmeter only 

 records the power consumed by the lamp. The amperage, assuming the same 

 number of watts as in the above example, would be found this: 1870 -=- 34 = 

 55 amperes. That is, with an inductor in place of a rheostat one could use 

 several times the amount of current and use only the same number of kilowatts 

 of power. As it is the power consumed that must be paid for, one can appre- 

 ciate the saving by using an inductor or choke-coil rather than a rheostat. 



The two cases just given are the only ones in which the wattmeter can be 

 used to find the amperage. If a current-saver, transformer, rectifier, or other 

 similar device is used in the circuit the amperage in the arc cannot be deter- 

 mined by the wattmeter, one must use an ammeter of the proper type for the 

 current. 



