484 ELECTRICAL MEASUREMENTS 



Each element is complete in itself and must be adjusted so 

 that it registers correctly at both high and low loads, as well 

 as at both unity and low power factors. It is essential, when 

 carrying out these adjustments, that both potential circuits be 

 kept energized, otherwise the moving element will experience an 

 abnormal retarding torque. 



Suppose the upper element to be under adjustment; both sets of 

 drag magnets are placed in what seems to be a reasonable posi- 

 tion and the adjustment is made as in a single-phase meter. 

 When it is completed, attention is given to the lower element, the 

 constant of which must be varied and made equal to that of the 

 upper element without changing the constant of the latter. 



It is not permissible to change the position of the drag magnets; 

 the change in the constant must be effected by altering the driv- 

 ing torque of the lower element. This may be done by varying 

 the fluxes and for this purpose taps are sometimes brought out 

 from the potential coil by which the number of active turns may 

 be altered. In the induction meters now made by the General 

 Electric Co., this adjustment is effected by changing the posi- 

 tion of the lower current coils by means of a screw. 



A very good check on the equality of the two elements may be 

 obtained by operating the potential circuits in parallel and the 

 current circuits in series and opposed; under these conditions the 

 disc should not rotate. 



When induction meters are used on loads which have a recti- 

 fying effect, such as three-phase arc furnaces, they must be in- 

 serted in the primary of the transformer which supplies the load. 



MERCURY MOTOR METERS 



The principle utilized in the mercury motor meters is that 

 illustrated by the familiar Barlow's wheel, in which a current 

 flows radially in a pivoted copper disc so placed between the 

 poles of a magnet that it is cut by the flux. On the passage of the 

 current the disc is set in rotation. 



The advantages of the mercury motor meter for direct current 

 are the elimination of the commutator, as well as the wire-wound 

 armature and the brushes, and the decrease of the wear on the 

 lower pivot and jewel. These things tend to decrease the expense 

 of maintenance. 



