POWER MEASUREMENT 



307 



the entire potential circuit is included. The corresponding 

 corrections when a small output is measured are obvious. 



Compensation for Energy Loss in the Potential Circuit. 1 

 In research work it is frequently necessary to measure small 

 amounts of power, a few watts, and in this case the loss in the 

 potential circuit of the wattmeter may be a large percentage of 

 the power to be determined. To avoid the necessity of making a 

 correction, instruments are sometimes so designed that this error 

 is compensated. In this case connection II is used. The current 

 which flows through the fixed coil is made up of two components, 

 one due to the load, the other to the current in the movable 

 coil circuit. The effect of this last must be compensated, for 

 the magnetic field in which the potential coil moves is due to 

 both components. If a second winding, coincident at all points 



3 Load 



FIG. 178. Showing wattmeter connections. 



with the regular current coil, could be put on the bobbin carry- 

 ing the fixed coil, and be connected into the potential circuit so 

 that its effect opposed that of the main-current coil, the com- 

 pensation would be exact; for instance, if the load circuit were 

 broken, the net ampere-turns acting on the movable coil would 

 be zero and there would be no deflection. As the two coils 

 cannot be made coincident, care must be exercised in placing the 

 compensating turns so that when the load circuit is broken, the 

 net magnetic field at the movable coil will be zero for all positions 

 of the movable coil. Otherwise the degree of compensation 

 will vary with the scale reading. There is a small transformer 

 action due to the mutual inductance of the two windings on the 

 fixed coil, but in commercial measurements at the usual voltages 

 this does not cause an appreciable error. 



This compensation may be extended so that the power lost 



