CHAPTER IV 



21. Theory of the wattmeter. Effect of self-inductance 22. Effect of capacity 

 23. Effect of eddy currents 24. Effect of mutual inductance 25. Cor- 

 rection for loss in wattmeter 26. Power measurement in three-phase circuits 

 27. Relations of line p.d.'s and currents 28. Case of balanced load. Tangent 

 formula 29. Power measurement by ammeters and voltmeters 30. Measure- 

 ment of energy. 



21. Theory of the Wattmeter. Effect of Self- 



inductance 



ONE of the most important measurements in alternating-current 

 circuits is the measurement of power. By power is here meant, as 

 explained in 7, the mean value of the power over a complete period. 

 Although numerous methods, some very ingenious, have been devised 

 for the measurement of power, it is nowadays generally admitted 

 that by far the most satisfactory method is that involving the use of 

 a wattmeter. 



The oldest type of this instrument is the electrodynamometer 

 wattmeter, which resembles in general construction the well-known 

 Siemens electrodynamometer for the measurement of currents. It 

 consists of two coils, a fixed and a movable one, arranged with their 

 planes at right angles to each other, the movable coil being provided 

 with a torsion head, by means of which it can always be restored to 

 its original position when deflected by an electrodynamic couple. 

 The fixed coil is placed in the main circuit, while the movable coil 

 is connected in series with a high non-inductive resistance, and is 

 joined across the two points between which power is being 

 measured. 



The connections are indicated in Fig. 31, where F denotes the 

 fixed or main or " current " coil of the instrument, M the movable or 

 shunt or "pressure" coil, r\ the high non-inductive resistance in 

 series with the pressure coil, and A the portion of the circuit in 

 which power is being measured. The current coil represents the 

 ammeter part of the instrument, and the pressure coil the voltmeter 

 part. The dotted line indicates an alternative mode of connection 

 for the pressure coil. 



