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| Figure 17-8 Dynamometer used as a wattmeter (Courtesy, General Electric Co.) |

| magnetic fields set up by both coils. These fields, in turn, are dependent on the currents flowing through them. In the simple wattmeter (Fig. 17-8), the magnetic field of the stationary coil is proportional to the line current, that of the moving coil is proportional to the voltage, and the torque is proportional to the product of the line current and voltage. Since this product represents the instantaneous value of power in the circuit, the instrument reads the average of the power pulses and hence can be calibrated in watts. |

| In an ac circuit, this instrument indicates real power, not the product of volts and amperes. The interaction of the magnetic fields is such that the instrument indicates the product of the voltage and that part of the current in phase with the voltage (Fig. 17-9). |

| Connections for a two-element wattmeter to measure a three-phase, four-wire load are shown in Fig. 17-10. |

| The stationary coil is sometimes provided with a laminated-core structure of magnetic alloy (Fig. 17-11). This type of element has a greater magnetic concentration in the air gaps, which produces a greater torque. Since it requires less power for a given deflection, it is therefore applicable to small portable instruments. |

| Reactive Volt-Ampere Meter or Varmeter |

| A wattmeter can be made to measure the reactive volt-amperes or vars in a circuit. Since the wattmeter indicates the product of the voltage and the component of the current in phase with the voltage, it is necessary to shift the voltage only 90° so that the current in the movable coil will lag that in the stationary coil by 90°. This measurement is accomplished by providing a |

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