40 TREATISE ON ALTERNATING CURRENTS. 



resulting current in an inductive circuit. The ordinates of curve 3 

 are the products of corresponding ordinates of curves 1 and 2, and 

 the curve shows the variation of the power. The straight line 4 in 

 the figure shows the mean power. A close study of these curves 

 will amply repay the reader. 



MEASUREMENT OF POWER. 



29. Since the power given to an alternating-current circuit 

 cannot be determined by the use of a single ammeter and voltmeter, 

 special means for its measurement have to be devised. 



Various methods involving combinations of ammeters and 

 voltmeters may be employed, but the simplest method of 

 measuring power is the use of a single instrument called a 

 Wattmeter. 



The wattmeter generally consists of two vertical coils of wire 

 arranged so that one is fixed and the other is capable of rotation on 

 a vertical axis. The two coils are placed so that their planes are 

 at right angles to each other. One of the coils, a thick coil of few 

 turns, is placed in series with the circuit in which the power has 

 to be measured, while the other coil, a long thin wire coil of high 

 resistance, is placed directly across the terminals of the circuit. 

 The thin wire coil has so high a resistance that the current passing 

 through it is very small, and should be arranged so as to have 

 as low a self-induction as possible, while the thick wire coil 

 carries the whole current passing through the circuit. 



The magnetic field due to the thick coil is therefore proportional 

 to the current passing through the circuit, and that due to the 

 thin coil is proportional to the P.D. between its terminals. The 

 mutual force exerted by the two fields when the planes of the coils 

 are at right angles is at any instant proportional to the pro- 

 duct of the current passing through the circuit and the P.D. 

 between its terminals. Thus the mutual action is at any instant 

 proportional to the power which is being given to the circuit at 

 that instant. 



The movable coil is suspended so that its geometrical centre 

 coincides with that of the fixed coil, and is attached to a torsion head 

 by means of which the coils may be kept with their planes at right 

 angles to each other, and the mutual force is measured by the angle 

 through which the torsion head has to be rotated to do this. 



