496 ELECTRICAL MEASUREMENTS 



of the meter under test, and then allow for the meter constants. 

 For example, denote by x the meter under test, and by s, the 

 standard meter. The average powers given by the two meters 

 are P x and P a ; then 



xx, 



*j * a 



t t 



The " percentage of accuracy " is ~ 100 = ^^" 



* \&h)t 



It would be an obvious convenience if the meters had equal 

 watt-hour constants. 



The advantages of this method are the elimination of the use 

 of the stop watch by the tester, independence of load and vol- 

 tage fluctuations, and the reduction of the working force, for 

 only one man is necessary. Independence of load and voltage 

 variations is a most decided advantage, for at times, especially 

 if high-capacity meters are being tested, it is necessary to use 

 the consumer's load, and this may be fluctuating. 



Rotary standards are now made for both alternating and direct 

 currents. 



The alternating-current standard is started and stopped by 

 making and breaking the potential circuit. In the direct- 

 current instrument the potential circuit is kept closed so that the 

 armature and disc rotate continuously; the register is thrown into 

 and out of gear by an electrically operated clutch. 



It is essential that the construction of rotary standards be 

 such that their accuracy will not be affected by the necessary 

 handling during transportation. This means that the geometry 

 of the coil system and of the brake must not alter, and that the 

 friction must remain constant. To insure this last it is neces- 

 sary to provide means for raising and clamping the movable 

 system so that the pivots and jewels may not be injured. 



To reduce the irregularities due to unavoidable friction, the 

 commutators used in direct-current standards should be of small 

 diameter and the brush pressure constant. A high ratio of torque 

 to weight of the moving element is most desirable. 



When the instrument is connected into the circuit, care should 

 be taken that the field and the armature coils are at practically 

 the same potential, especially if the voltage is so high that a 

 multiplier is used. 



