One of the many items you are not permitted to bring aboard aeroplanes is a mercury thermometer (or more accurately “mercurial thermometers and barometers”). Why?
Aeroplanes are made largely of aluminium, as it has one of the best available strength-to-weight ratios.* When aluminium is exposed to air it forms a tough coating of aluminium oxide that doesn’t flake away like iron oxide (rust) does and which prevents chemicals from reacting with the aluminium.
But if the raw elemental aluminium is exposed (e.g. by a scratch) and comes into contact with mercury it forms an amalgam, tearing away at the aluminium and causing it to lose its structural integrity. As the aluminium is eaten away it combines with the air to form aluminium oxide and falls away (as seen in this video). This allows the mercury to reach fresh aluminium and the process then repeats, so a small amount of mercury can do a large amount of damage. If a mercury thermometer were to leak aboard an aeroplane the aeroplane would need to be taken out of service and disassembled to assess the damage the mercury might cause. There have been at least two incidents in which aircraft exposed to mercury have been written off by their insurers.
The forty second timelapse video above shows the effect of a small amount of mercury on an aluminium I-beam over the course of two hours.
* Titanium has a better strength-to-weight ratio (288 kNm/kg compared with aluminium’s 214 kNm/kg) but it costs about five-and-a-half times more. Titanium is used in jet engines, where strength-to-weight ratio is key.
As previously mentioned, size matters when it comes to telescopes: the bigger the mirror, the better the telescope (i.e. the greater the resolution and light-gathering capability). The world’s largest single telescope mirrors are the 8.4-metre telescopes used by the Large Binocular Telescope.
Telescope mirrors must be perfectly polished and accurate in shape and size to within a billionth of a metre. This means that making telescope mirrors is extremely difficult and therefore time consuming and very expensive, costing millions of dollars.
But there is an alternative to polished metal and silvered glass: mercury. Simply filling a pan with liquid mercury will create a perfectly flat* reflective surface; the surface can then be made curved by spinning the pan. No expensive manufacturing is required and this keeps costs low: a liquid mirror costs about 1% of the cost of a similar-sized conventional mirror. (They do have the disadvantage that they can only point upwards, however.)
The University of British Columbia’s six-metre Large Zenith Telescope in Vancouver is the world’s largest liquid mirror telescope.
The simplicity of constructing a liquid mirror telescope has even led to suggestions that one should be built on the Moon.
* To give you some idea of how flat a liquid mirror is: if you built a mirror the size of the Earth, the largest bump would be less than a millimetre in height.
Whilst planning a lesson yesterday I discovered that there aren’t many good photographs of objects floating in mercury on the web. This post is an attempt to rectify this problem.