64 



ALTERNATING CURRENTS 



flux must lag the line voltage by 90. As it is impossible to make 

 the resistance of the potential-coil zero, its current will lag by an 

 angle less than 90. At low power-factors this introduces con- 

 siderable error in the meter registration. However, by properly 



adjusting the resistance R, 

 the potential-coil flux may 

 be brought into the 90 

 relation and the meter will 

 register substantially cor- 

 rectly at all power-factors. 

 To adjust the compen- 

 sation, the meter is made 

 correct at unity power- 

 factor and then the power- 

 factor is dropped to some 

 low value, as 0.5. If the 

 registration is now in error, 

 it is due to improper com- 



FIG. 61. Diagram of induction watthour pensation. The meter IS 



again made to register cor- 

 rectly by changing the resistance R, the two small wires of this re- 

 sistance being either twisted or untwisted and then soldered. If 

 the meter under-registers when the load current lags, the resistance 

 R should be decreased; if the meter over-registers with lagging cur- 

 rent the resistance R should 

 be increased. The reverse is 

 true with leading current. 



L is a small metallic stamp- 

 ing placed under the potential 

 lug and can be moved later- 

 ally by means of the lever K. 

 Its function is to provide the 

 small torque just necessary 

 to overcome the friction of the meter. The operation of this 

 adjustment is as follows: 



Figure 62 shows the stamping under the lug, set off center. 

 When the flux starts to pass down through the lug a current is 

 immediately induced in the short-circuited stamping. This 

 current, by Lenz's law, opposes the flux entering the stamping so 



Eddy 

 Current 



FIG. 



. Shaded-pole principle of the 

 light-load adjustment. 



