THE CALIBRATION OF INSTRUMENTS 601 



springs and the copper movable coil. Both instruments and 

 multipliers should be properly ventilated. 



Stray Fields. One of the most troublesome sources of error 

 in industrial testing is due to the stray fields from busbars, 

 feeders, motors, masses of iron, etc. These may so modify the 

 strength of the field in which the movable coil swings that the 

 indications of the instrument are entirely untrustworthy. 

 Especial care must be exercised when working near switchboards. 



Stray fields due to ordinary working conditions are not likely 

 to produce permanent alterations in the instruments, but vio- 

 lent short-circuits may cause permanent changes in the strength 

 of the magnets and occasion very large errors. This is especially 

 true of direct-current watt-hour meters. In this case the stray 

 field is that due to the current coils. Fig. 266 shows the normal 

 distribution of magnetism in the drag magnets as well as the 

 distribution after a short-circuit. Strong alternating stray fields, 

 due to short-circuits, may also greatly modify the strength of any 

 permanent magnets in their neighborhood. If such an accident 

 has happened with either alternating or direct currents, no reli- 

 ance should be placed on the instruments until they have been 

 tested and found to be correct. 



Direct-current stray fields of ordinary strength cause a per- 

 centage change throughout the scale in the indications of moving- 

 coil ammeters and voltmeters. Of course, they produce no effect 

 on alternating-current instruments. Alternating stray fields of 

 ordinary strength have no effect on direct-current moving-coil 

 voltmeters and ammeters, but will affect ammeters, voltmeters, 

 and wattmeters in which the current is of the same frequency 

 as the field. 



The effect on dynamometer instruments will depend on the 

 angular position of the movable coil with respect to the direction 

 of the stray field, being a maximum when the plane of the coil is 

 in the direction of the field and zero when it is perpendicular 

 to it. Assuming that the stray field is fixed in direction, dyna- 

 mometer instruments with torsion heads should be set up in 

 such a position that the movable coil is perpendicular to the field. 

 The proper position is found by sending full current through 

 the movable coil alone and turning the entire instrument in 

 azimuth until the deflection disappears. 



