DRYSDALE WATTMETER 



ZERO ADJUSTMENT (CKTW 



for compensating the couple due to an external field. In order to 

 remedy this defect, Lord Kel- 

 vin has introduced an astatic 

 type of wattmeter, the latest 

 form of which is shown in 

 Fig. 51. The movable system 

 consists of two coils, similar 

 in every respect, but connected 

 up so that the current circu- 

 lates round them in opposite 

 directions. Thus the couples 

 exerted by a uniform external 

 field on the movable coils are 

 in opposite directions, and 

 balance each other. Corre- 

 sponding to each movable 

 coil there is a fixed coil 

 (wound in two sections, one 

 on each side of the axis of FIG. 52. Drysdale Wattmeter, 



rotation). Oscillations are 



prevented by an oil-damping arrangement precisely similar to that 

 employed in the multicellular type of voltmeter, and the method of 

 suspension is also identi- 

 cal with that used in 

 this latter type of in- 

 strument. 



An astatic arrange- 

 ment of movable coils, 

 though rendering the 

 reading of the instru- 

 ment independent of a 

 uniform, external field, 

 would still be liable to 

 disturbance by a field of 

 varying intensity. In 

 general, however, it is 

 possible to place the 

 instrument so as to 

 render the error due to 

 non-uniformity of field 

 very small. FIG. 53. Drysdale Wattmeter.* 



An extremely in- 

 genious form of wattmeter, which possesses the double advantage 



* The block for this illustration has been supplied by makers of instrument (Messrs. 

 Nalder Bros. & Thompson, Ltd.). 



