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| 2. Most instruments depend on the interaction of two or more magnetic fields. These fields may be created by electromagnets or by permanent magnets (Fig. 17-1). By combining the fields measuring current and voltage, instruments may determine values of resistance, power, energy, power factor, and other quantities in addition to the basic ones of current and voltage. |

| 3. Instruments comprise ammeters (Figs. 17-2 and 17-3), voltmeters (Fig. 17-7), wattmeters (Figs. 17-8, 17-10, and 17-11), volt-ampere meters, power-factor meters (Figs. 17-12 and 17-13), frequency meters (Fig. 17-14), watt-hour meters (Figs. 17-15, 17-17, and 17-18), volt-ampere-hour meters (Fig. 17-21), demand meters (Fig. 17-22), and other special purpose meters. |

| 4. Meters may be of the indicating type, maximum reading type, recording type (Fig. 17-23), or clamp-on type (Fig. 17-24). |

| 5. Ammeters and current elements of other meters are connected in series with the circuit (Fig. 17-25) or equipment under observation. Voltmeters and voltage elements are connected in parallel. |

| 6. The range of instruments for measuring current values may be increased by the use of current transformers for ac and by shunts for dc (Fig. 17-26). For voltage values, potential transformers are used for ac and potentiometers for ac and dc (Fig. 17-27). |

| 7. For measuring resistance, ohmmeters (Fig. 17-28A and B), Meggers (Fig. 17-28C), and Wheatstone bridges (Fig. 17-29) may be used. |

| 8. Phase rotation indicators (Fig. 17-30) show the rotation or sequence of phases in a polyphase circuit, and are used when connecting such circuits or equipment in parallel. |

| 9. Instruments may be of the portable type or of the stationary or mounted type. |

| 10. All instruments should be handled very carefully and generally used or installed away from magnetic fields which may affect their operation. |

| STUDY QUESTIONS |

| 1. What is the principle of operation of an ammeter? Of a voltmeter? How are they connected in a circuit? |

| 2. Describe two types of instruments for measuring current. What is the difference in their mode of operation? |

| 3. What other instruments employ the electrodynamometer principle? |

| 4. What is a wattmeter? A watt-hour meter? What is the difference between them, and how are they connected in a circuit? |

| 5. What is the principle of operation of a watt-hour meter? What other quantities may be measured by this type of instrument? |

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