Mercury arc rectifiers are the predecessor of the Cooper Hewitt lamp, so I think maybe some of my questions can be answered here.

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1) Gas Fill / No Gas Fill:
I am aware that Cooper Hewitt lamps contain only mercury vapor, and have no starter gas. Is this the same for mercury arc rectifiers as well?

2) Pressure:
What is the approximate operating pressure of one of these? I assume it varies depending on what properties are needed for the rectifier but a general idea would be interesting to know.

3) Anode Material:
I have heard of the anodes being made of carbon, and I have heard of them being made of iron. Is there any situation where one is better than the other? I am assuming that electrodes made of other metals would not be suitable due to possible amalgamation with mercury, but again I have no idea.

4) Operating Principle:
I am assuming that the bright blue dancing dot that can be seen in the pool of mercury is a hot spot that acts like a hot cathode and emits electrons. Is this correct? I haven't heard anyone thoroughly explain this yet.

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Thanks!

To answer some of your questions, there are several types of diodes and rectifiers for certain purpose.  I have a few examples in my collection that uses certain applications:

RCA 866A Mercury Vapor Half Wave Rectifier for power supply.

VT-266 Half Wave Vacuum Rectifier for high voltage power supply.

RCA Tube 3A3C High Voltage Diode for flyback colour tube circuit.

RCA 3B28 Half Wave Xenon Filled Rectifier for high voltage power supply.

If you wish, you can find more of them at Vacuum Tubes.

@rjluna2
Those are standard vacuum/gas filled rectifiers, the 866A is mercury filled, but that is not a mercury arc rectifier. Mercury vapor rectifiers work very differently to mercury arc rectifiers, because mercury vapor rectifiers have filaments.

If you Google a picture of a mercury arc rectifier, it is one of those large things with a big glass bulb and a pool of mercury on the bottom, with arms on the sides for the electrodes.

Oh, I see what you mean.

https://en.wikipedia.org/wiki/Mercury-arc_valve