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| Principle of Operation |

| The general principle of operation of all rectifiers is that the action of the device permits current to flow freely in one direction but presents a great resistance to its flow in the opposite direction. There are many types of rectifiers, but two are used extensively: the mercury-arc rectifier for large loads and the contact or disc rectifier for smaller loads. |

| Mercury-arc rectifier. The mercury-arc rectifier consists of a metal or glass chamber with a pool of mercury at the bottom and one or more anodes made of graphite or iron supported above the surface of the pool at a proper distance (Fig. 16-7). A vacuum pump maintains a low pressure within the chamber. A movable auxiliary starting anode is used to strike an arc, which produces mercury vapor from the pool. The ionized mercury fills the chamber, and gaseous conduction is established between the mercury pool and the anodes. This conduction tends to sustain itself as long as the current does not drop to zero and a positive potential (with respect to the mercury pool) is maintained. |

| Figure 16-7 Tank-type mercury-arc rectifier (Courtesy, General Electric Co.) |

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