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Monthly Bulletin
May/June 2005 - Vol 8 No. 3


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Pharology 101 - Mercury float

by Denise Shultz, LoA Inc President & Prism Editor & Paul Shultz

Montague Island Lens
The original Montague Island Lens, now on display at the Narooma Maritime Museum, sits on a mercury float

Photograph: Kristie Eggleston

We begin a series of "lessons" for lighthouse buffs (pharophiles) on lighthouse technology, physics and chemistry, as written and researched by our LoA Inc President, Denise Shultz, who in a former life was a chemical engineer. This first part of Pharology 101 discusses the mercury float, which was originally used in most lighthouses around the world to support and rotate the lens.

Mercury (Hg) is an element which plays vital role in the lighthouse design. No 80 in the Periodic Table, it is one of the heaviest metals found naturally on Earth, and is surpassed only by lead, bismuth and the radioactive elements like uranium, in its atomic weight. Uniquely, it occurs in liquid form at normal temperatures and has to be cooled down to -39ºC to solidify. Also called quicksilver, it was widely used in mining ( gold and silver form amalgams with mercury and can therefore be separated from the ore by being “dissolved” in it), medicine (mercury compound calomel Hg2Cl2 was used as an antiseptic) and agriculture (pesticide and fungicide). Mercury and especially its organic derivatives and soluble salts are highly poisonous when ingested, inhaled as vapour or even absorbed by contact with the skin.

Chance Brothers of Birmingham diagram of a lighthouse lens
Chance Brothers illustration showing a mercury float
This illustration shows the design of a lantern complete with a cross section on the left showing the mercury float.

Photograph: Chance Bros 1910

Since the end of the 19th century, mercury was also found in virtually every lighthouse designed to carry a rotating large Fresnel lens. It was for a different reason altogether.

To rotate a very heavy crystal glass optic apparatus, a mechanism was needed that offered the smallest resistance while taking the least amount of space. A mercury float turned out to be the perfect answer.

For an object to float, it has to either be lighter ( to have a lower density) than the medium it floats in, or has to be designed in a way that the weight of the medium it displaces when immersed is larger than its own weight (Archimedes Principle). A piece of wood or plastic floats in water because it’s mean density is lower than 1000kg/m3 (which is the density of water). But the ship made of iron floats in the seawater despite iron being almost eight times denser than water. In this case, the Archimedes law is being put to use. The hull of the ship is shaped in such way that it displaces a very large volume of water. If all this water was weighed, it would surely be much heavier than the ship, its cargo included. Otherwise the ship would sink.

The mercury float in the lantern room of a lighthouse works on similar principle.

A circular (O shaped) basin forms a container into which is fitted the vessel that carries the optical apparatus. The basin’s outer diameter is usually around 2m. The optic carrying vessel is shaped like a hollow cylinder which fits inside the slightly larger circular trough. With no liquid, the vessel would just sit on the bottom of the bath, and the rotation would be impossible.

When the trough is flooded with liquid the liquid starts to buoy the vessel. It is just the question of pouring the right amount of liquid in the trough to give it enough buoyancy to float the vessel. This critical volume could be quite large depending on the liquid medium. 

Mercury float diagram
Vertical cross-section of a mercury float
The pedestal which supports the first order revolving mechanism could be screwed down for maintenance. The circular trough is filled with mercury into which the floating vessel is immersed. The optical apparatus sits on top of the vessel.

Diagram: Paul Shultz

According to Chance Brothers, the first order lens with its support and other associated parts would weigh 4318 kg. If water was used, the volume of the basin would have to be also at least 4318 litres, enough for the water to buoy the apparatus. That would mean a circular trough with the diameter of 2m would have to be 3.3m deep.

Not only is such a deep float hard to accommodate in a confined space of a lighthouse, but from engineering point of view, such a mechanism would be quite impossible to mange because it would be prone to jamming. 

Floating the apparatus on oil could be another option, but as is commonly known, oil is even lighter than water and the whole apparatus would have to be even larger. True, there would not be any corrosion to cast iron, but the presence of so much flammable material so close to open flame would be another detriment. Remember, there was no silicon oil in 19th century.

Despite its poisonous nature, mercury shaped up to be the best medium. Not only it was not corrosive to cast iron, it was also non-flammable, in fact, almost inert to any chemical reaction at normal temperature. Best of all, with density of 13 600 kg/m3, it is 13.6 times heavier than water and as a result, for the same 4.318 tonne optic, instead of using 4318 or more litres of water or oil, only 80 litres of mercury would do the same job. That cuts the depths of the 2m diameter float to a much more manageable 25cm.

Mercury float is a very elegant solution to a huge problem of how to fast revolve heavy optics without wasting energy on overcoming the friction. The whole mechanism could be easily turned with a gentle push. Since the invention of the frictionless mercury float, the weights that turned the optical apparatus could be much reduced, making the keeper’s job of winding them back up so much easier. 


Alan Daniels (Cape Leeuwin)
A Few Notes on Modern Lighthouse Practice (1910) Chance Brothers and Co. Limited, Lighthouse Engineers and Constructors, Birmingham

Email Denise Shultz

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