306 ELECTRICAL MEASUREMENTS 



for it depends on the geometry of the system of coils, that is, 

 on the angle between the coils, and this angle varies with every 

 change of load. In consequence, the scales of portable watt- 

 meters are generally non-uniform. Proper proportioning of the 

 relative diameters of the fixed and movable coils will do much 

 toward correcting this, see page 80. 



Fig. 177 shows, in section, one form of portable wattmeter 

 which is in common use. 



Potential TermijTob /^Current Terminal 



Series 



Resistance 



edCoil 



V;!* u Damping Chamber \.. , - ., 

 omela Set Pag? it MovabkCoil 



FIG. 177. Phantom view showing construction of Weston portable 



wattmeter. 



Heating Losses in Wattmeters. It is important to note just 

 what a wattmeter measures. P in formula (1) is the power 

 given to the circuit by the current which flows through the 

 fixed coils, that power being expended between the two points 

 at which the potential terminals are attached to the circuit. P 

 therefore must include the heating loss either in the current 

 coils or in the potential circuit, depending on whether the point 

 a (Fig. 176) is on the supply or the load side of the current coils. 



Where the total amount of power is small, it may be necessary 

 to correct the readings for the loss in the instrument itself. 

 The two possible methods of connection are given in Fig. 178. 



With connection I the indication of the wattmeter includes 

 the PR loss in the current coils; with connection II the loss in 



