CHAPTER XVII. 



Measurement of Power The Wattmeter Three-voltmeter Method Three- 

 ammeter Method A Dynamometer Method Measurement of Power in 

 Tri-phase Circuits. 



THE MEASUREMENT OF POWER IN ALTERNATING-CURRENT 

 CIRCUITS. 



129. We have seen ( 28) that the power given to an alter- 

 nating-current circuit is ei cos 9 where e is the R.M.S. potential 

 difference between its terminals, i the R.M.S. current passing 

 through it, and the difference of phase between the current and 

 potential difference. 



It is obvious, therefore, that the power cannot be measured by 

 means of an ammeter and a voltmeter, since the product of their 

 readings is only ei. 



13O. The Wattmeter. The instrument most generally 

 used for the measurement of power is called a Wattmeter, and, 

 as explained in Chap. V., consists generally of two coils of wire 

 placed symmetrically with regard to each other, with their respec- 

 tive planes at right angles to each other. One coil, usually the 

 larger one, is wound with thick wire and is fixed in position, 

 while the other coil, wound with thin wire, is suspended inside 

 the thick wire coil, and is attached to a torsion head by means 

 of a silk thread and a fine phosphor-bronze spiral. The thick coil 

 is placed in series with the circuit, the power given to which is 

 to be determined, and the thin coil is placed in parallel with the 

 circuit. The thin coil should have a very high resistance so as 

 to take a negligibly small fraction of the current in the main 

 circuit, and should have as small a self-induction as possible. 



When the wattmeter is in circuit the movable coil rotates 

 through an angle depending upon the product of the field strengths 

 due to the currents in the two coils. The thin coil is brought 



