The two auxiliary electrodes are connected to opposite ends, each via a Carbon resistor.
The main electrodes are connected to the mains, through a current limiting device, such as an inductive resistor or ballast, to limit the current of the lamp. Without such a current limiting device, the lamp gets destroyed on start, because the current tends to go to infinity as all gas discharges tend to draw more and more current, eventually causing a runaway current scenario.
When the lamp is properly wired, the existing voltage initiates an Argon glow discharge between the main and the auxiliary electrode pair(s), causing a sufficient number of Mercury atoms to be ionized. This in turn initiates a low pressure discharge between the two main electrodes.
The discharge gradually transforms from low to high pressure, elevating the pressure and temperature of the Mercury vapor inside the inner discharge tube, to roughly 7-10 atmospheres and ~1,600 degrees Celsius, respectively. (That's why it's necessary to construct the inner discharge tube from fused silica, which has a melting point around 1,700 degrees Celsius). Under these conditions, the emission lines of Mercury are thermally broadened and partially self-absorbed.
The lamp emits all the visible lines of Mercury and a little continuum. The radiation from the inner tube is particularly dangerous to eyes and skin, as such radiation includes lines in the shortwave ultraviolet and can cause severe eye and skin burns, including conjunctivitis and skin erythema. Prolonged exposure to such radiation, may cause blindness and skin cancer, as it causes alterations to the DNA of epithelial skin cells. NEVER operate a Mercury vapor lamp, if the outer bulb is broken.
For the above reason and for other reasons, such as thermal insulation, the inner tube is enclosed in an outer evacuated bulb usually made of hard borosilicate glass. The outer bulb effectively absorbs all shortwave ultraviolet radiation below ~280-300nm, and as such the lamp's light becomes somewhat more benign. Depending on the outer glass type, the lamp still emits lots of longwave ultraviolet radiation and can still cause eye irritation after long exposures near the lamp.
On the first and second photos the lamp can be seen operating, with the Mercury arc forming between the two main discharge tube electrodes.
The color of the High Pressure Mercury arc is characteristic green/bluish white. Although the intensity of all the visible Mercury lines in the arc is such as to make the lamp's light appear green/bluish white, the lamp's spectrum is severely lacking in red emissions. For this reason, newer types of this lamp have the inner surface of the outer bulb coated with fluorescent phosphors, which utilize the longwave ultraviolet radiation of Mercury and produce additional red emissions. Such a lamp is shown on the third photo, above.
The basic clear outer bulb Mercury vapor lamp is still used in the US, usually in porch security light installations. In Europe the clear Mercury vapor lamp is not in use anymore and the type shown above is quite hard to find, unless one orders such lamps in large quantities.
The Mercury vapor lamp has evolved considerably, with newer lamps containing additional ingredients in their discharge tubes, to improve the emitted light. These are called Metal Halide lamps. Their color is very much improved, but their control gear is quite complex and they are quite expensive.
In terms of lifetime, the Mercury vapor lamp is still the undisputed champion. There are Mercury vapor lamps in my neighborhood in Athens, which I remember since I was 8 years old that are still functional.
For some strange reason, I was completely mesmerized and magnetized by the strange color hue of this lamp. Reasons unknown. As I am also nearsighted, the lamp also had a strange appearance through the edge of my glasses. Regular eyeglasses are non-achromatic, so if you wear corrective lenses for nearsightedness and you look at a light source through near the edge of the lenses, you see a small spectrum of the light source. The Mercury lamp had a strange bluish halo when I was looking at it near the edge of my glasses.
It turns out that this "halo", is nothing more than the blue Mercury spectral line, at 436nm. The color of this line is a mysterious Cobalt-blue, very pleasant and slightly off focus, when looked at through glasses.
I subsequently bought a Mercury lamp and a ballast from an electrical contractor and started playing with it. When the lamp was starting, it produced a very pleasant pink glow, which gradually transformed into a bright cool white light, after the lamp completely warmed up.
One day while starting up the lamp, I took a peak under the lamp's white coating. I had no idea what the white stuff under the bulb's surface was, but noticed that whatever device was hiding under the outer bulb, produced light which was quite different from the light of the entire lamp, seen from the outside of the white coating. The striking contrast between the greenish-blue inner light and the pink outer glow, intrigued me. This was the start.
I started searching for ways to see more. I acquired a set of cheap prisms and started investigating the spectrum of this lamp. I was 13 years old at the time. My father saw this interest and although he was away from home (because my parents had been divorced around 1972), when he died in 1986, I found this small brand new hand-held spectroscope in his personal belongings. Thank you Father.
The culmination of my efforts to investigate the Mercury vapor lamp, led to a large collection of lamps based on the criterion that each member of this collection should have a different spectrum.
Mercury expresses most of my character's traits very well, so I also developed a strange attraction to the element Mercury, itself. I acquired substantial quantities of it and still keep some for personal use. I also have a mercury pendant. Essentially all that I am (from a scientific standpoint) is the result of my fascination with the mercury vapor lamp.
Here is my latest spectroscopic investigation into the world of light sources, over a period of almost 30 years. During these years, technology changed, most Mercury vapor lamps in my neighborhood were replaced with Sodium vapor and Metal Halide lamps, but my first fascination with the original Mercury vapor lamp, remains alive to this day.
If you want to see Grotrian energy level diagram details for mercury, consult NIST.