These wee steel bars are known as riders, and they mark the travel extent of the Mercury column. They are held in place by a feeble magnet. When the reset button is pushed, it moves the magnet away from the tube, and the riders will settle.
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The most common steel is mild steel, also called plain-carbon steel. It's density is approximately 7.85g/cm3. (Wikipedia)
A pound of steel.
Ah, what a lovely question! Structural steel is measured by weight, not area. However, if you're looking to cover an area with steel, it depends on the thickness of the steel sheets. I encourage you to consult with a steel supplier or engineer to determine the amount of steel needed for your project. Remember, there are no mistakes, just happy little accidents in the world of creativity!
Steel.
There is no single diameter - there are steel rods of different diameters for different uses.