Page 31 - Curriculum Visions Dynamic Book

 A mercury thermometer.
The mercury thermometer
All metals expand and contract considerably as the temperature
rises and falls. Mercury has the added advantage of being a liquid. This means it can be placed in a narrow tube.
By confining the mercury to a narrow (small-bore) tube, the changes in volume of the mercury can be made to show up more easily.
Other liquids expand and contract with changing temperature. Water and alcohol could both be used in thermometers. However, mercury has a wider range of use: it does not freeze until the temperature falls to nearly -40°C, and it does not boil until 357°C. By contrast, water freezes at 0°C and boils at 100°C.
 Towards the end of the 19th century, mercury baths were used to float the lamp housing in lighthouses.
meniscus: the curved surface of
a liquid that forms when it rises
in a small bore, or capillary tube.
The meniscus is convex (bulges upwards) for mercury and is concave (sags downwards) for water.
Lighthouse lamp bearings
Mercury has been used widely inside lighthouses since the end of the 19th century. Before this time lights were fixed. But by floating the lights in a tank of mercury, the lights could be made to rotate quickly with almost no friction and using just a small motor.
The process of floating lights on mercury led to a new way of using lighthouses. Rotating beams could be made to shine a unique pattern of light. This enabled sailors to identify each lighthouse by its pattern of beams.
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