yo 



ALTERNATING CURRENTS 



plates are shaped so that as the movable system is rotated the 

 distance between the cylinders and the fixed plates decreases. Owing 

 to the buoyancy of the movable system due to the oil immersion, 

 the frictional error is said to be very small. 



36. Wattmeters 



Owing to the inconvenience of having to make an adjustment 

 before taking a reading, the zero type of dynamometer-wattmeter 

 has practically disappeared (except for laboratory purposes), all com- 

 mercial types of wattmeter being direct-reading instruments. The 



MOVABLE COIL 



^-BOBBINS FOR FUED COILS 



MOVABLE COIL 



DAMPING VANE 



FIG. 50. Indicating Wattmeter. 



FIG. 51. Kelvin Astatic Wattmeter. 



usual arrangement of fixed and movable coils is shown in Fig. 50. The 

 current is led into and out of the movable coil by means of two hair- 

 springs of phosphor-bronze, which also furnish the controlling couple. 

 A wattmeter of the simple form shown in Fig. 50 is subject to 

 the error due to stray magnetic fields, as there is no arrangement 



