84 



ELECTRICAL MEASUREMENTS 



An astatic instrument can be set up without regard to the 

 earth's field and calibrated with direct currents. The curves so 

 obtained are immediately applicable to either direct- or alter- 

 nating-current measurements. 



In the instrument shown in 

 Fig. 43, the damping is ob- 

 tained by means of mica 

 vanes moving in the closed 

 chamber at the base of the 

 instrument. 



The Irwin Astatic Electro- 

 dynamometer. The Irwin 

 astatic instrument is inter- 

 mediate between the electro- 

 dynamometer and the current 

 balance. The fixed turns are 

 in the form of two coaxial 

 circular coils of the same di- 

 ameter, through which the 

 current circulates in opposite 

 directions, thus producing be- 

 tween the faces of the coils a 

 field which is directed radially 

 outward. The movable mem- 

 ber consists of two semicir- 

 cular coils mounted on a thin 

 disc of mica, the directions 

 of the currents being as in- 

 dicated. The straight sides 

 are as nearly as possible in 

 the axis of rotation, the 

 curved sides move in the field 



FIG. 43. Astatic electrodyna- 

 mometer. 



between the two fixed coils, 

 being attracted by one and 

 repelled by the other. This 

 iructioB brings the two movable coils very near together, 



wh lc h is advantageous as it reduces any effect which may be^ 



due to non-uniformity of the local or stray field 



