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| 5. A capacitor consists of two parallel plates that are conductors, separated by insulating or dielectric material. The electrostatic interaction between them produces effects that are in opposition to those produced by magnetic fields and hence, tend to cancel them. They are rated in volt-amperes, or more practically, in kilovolt-amperes, or kVA. Little or no maintenance is required. |

| 6. A rectifier is a device that converts ac to dc. The device generally permits a free flow of electricity in one direction and a great resistance to its flow in the opposite direction. |

| 7. In the mercury arc type (Fig. 16-7), vaporized mercury provides the conducting path which permits current to flow only in one direction. When the ac supply is single-phase (Fig. 16-9), the current drops to zero twice in each cycle and difficulty may be experienced in sustaining the arc. With a three-phase supply connected for six-phase operation, twelve such paths produce a more even flow of direct-current (Fig. 16-10). Pumps and other auxiliary equipment require regular maintenance. |

| 8. For smaller amounts of energy conversion, contact or disc type rectifiers may be used. Here the alternating current is made to pass through the contact surfaces of two materials, much more in one direction than the other (Fig. 16-11). The materials usually employed are copper oxide and selenium. These type units require practically no maintenance. |

| STUDY QUESTIONS |

| 1. What two effects are produced when a current flows through a wire? |

| 2. What is meant by inductance? What is the effect of inductance on the relation between voltage and current in an ac circuit? |

| 3. What is the function of reactors? How are they constructed and where are they used? |

| 4. What is meant by capacitance? What is the effect of capacitance on the relation between voltage and current in an ac circuit? |

| 5. What is the function of a capacitor? How are they constructed and where are they used? |

| 6. What is meant by impedance? Power factor? Resonance? |

| 7. What is the function of a rectifier? Where is it used? |

| 8. Name two basic types of rectifiers. What are their principles of operation? |

| 9. What are ignitrons? How are they controlled? |

| 10. What precautions should be taken in connecting and disconnecting reactors? Capacitors? Rectifiers? |

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