Electrical Engineering ⇒ Topic : Mercury Arc Rectifiers

Mercury Arc Rectifiers

Mercury Arc Rectifier (Simplest Form)

Refer Fig. (a) A mercury arc rectifier in its simplest form consists of an evacuated glass vessel with a pool of mercury at the bottom, acting as a cathode and at the top is anode (made of graphite).In the mercury pool is dipped a pointed electrode of semi-conducting material Boron carbide called the ignited or auxiliary anode. When the rectifier is fired, for a short duration current pulse is applied between the auxiliary anode and cathode which heats the surface of mercury and produces certain initial electrons. The high positive potential applied to anode attracts these electrons. The electrons reaching anode takes the form of an arc which starts at the anode and it is heated up subsequently.

                                                                                Fig.(a). Simplest form, of mercury arc rectifier

There is a luminous column next to anode which gives light. After the column there is a dark gap followed by a white hot cathode spot which travels irregularly over the mercury pool with high speed. The rectifier's cathode can only emit these electrons and is unable to receive them. Thus in Fig. (b), if the battery terminals are reversed making the anode negative and cathode positive, the electrons will be repelled and no current will flow. Thus it has a valve action which allows current to flow only in one direction, i.e., from anode to cathode. If the battery is replaced by an A.C. supply there will be current flowing only during the + ve half of the cycle.

                                  Fig.(b). Glass bulb containing gas and two electrodes against which potential difference is applied.