508 



ELECTRICAL MEASUREMENTS 



adjustable resistor r, to 6. The main portion flows through the 

 ''coarse coil" C of the multiplier, which is in this case a straight 

 bar, then through the resistor R, which is of such a magnitude as 

 to give the voltage drop required in the standard meter circuit. 

 The fields due to the currents in M M r and C are opposed, and in 

 the resultant field is placed an astatic movable-coil system, which 

 is provided with pivots and a damping device. The movable 

 member, in series with a suitable resistor, is placed across the line, 

 and serves to show when the fields due to C and MM' are bal- 

 anced. The adjustment is made by varying r. The ratio of the 



currents -f- = K, which is necessary for a balance, is determined in 



i'L 



the laboratory, therefore the corrected watt-hours by the test 

 meter are obtained by multiplying its indications by K + 1. 



The standard meter is set up where it will be as free from stray 

 fields as possible. The leads to it are flexible and readings are 



Dlff.M.V. 



FIG. 296. Arrangement for testing large direct-current watt-hour meters 

 by use of two shunts and a differential millivoltmeter. 



taken with the meter in four different azimuths 90 apart. This 

 is usually sufficient; but conditions may arise where, owing to 

 the change in the distribution of current between feeders which 

 are at different distances from the test meter, this procedure 

 would not eliminate the stray field errors. In such cases, a 

 shielded instrument is desirable. The multiplier being astatic, 

 with the centers of the upper and lower coils 2.5 in. apart, is 

 not affected by uniform stray fields. Anything that produces a 

 non-uniform stray field for instance, a busbar close to the in- 

 strument might, however, lead to a misinterpretation of the 

 balance. So the apparatus should be set up at some distance 

 from the switchboard. Fig. 296 shows a method where the 



