Mercury-Arc Rectifiers

When the bulb of Fig. 6 is tipped for starting, contact is made between the starting electrode and the mercury-pool cathode. When the bulb is restored to vertical, the contact is broken and the small spark that results fills the tube with positive and negative mercury-vapor ions. Electrons (the negative ions) flow to the anode that is positive at that instant, ionization by collision results, and additional ions are formed. For conduction to continue, electrons must leave the mercury-pool cathode. It is possible that the massive positive ions, slowly moving to the cathode, form a dense "cloud" of positive electricity very close to the pool and that this greatly assists in the liberation of electrons.

As the polarity of the input voltage changes, the electron current shifts from one electrode to the other, but, as indicated in Fig. 6, the conventional current always flows up through the battery to be charged. The reactor keeps the current from falling to zero and thus holds the arc.

Sometimes a single anode is placed in a mercury-arc rectifier called an Ignitron. A starting electrode, called by the trade name Ignitor, is employed to "fire" the tube each cycle it is to conduct. This electrode is of some semiconducting material, such as silicon carbide, and one end is permanently immersed in the mercury pool. A small current, when passed from this electrode to the pool, causes the formation of small sparks at the surface of the pool, and these sparks start conduction.