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Wall Thermometers

Wall thermometers have taken the thermometer from a simple scientific instrument and made it much more varied and decorative. This has been done without compromising the fundamental function of a thermometer: reading the temperature of the surrounding area. The first thermometers were not actual thermometers at all, they were thermoscopes. The basic difference between a scope and a meter is easily illustrated. While a speedometer is something that measures speed, a microscope just magnifies whatever you put in front of it. the first instruments to detect heat weren’t measuring temperature, because the concept of temperature had not been developed beyond such vague wording as hot, tepid, cool, or cold.

The scale of temperature was invented by one Daniel Gabriel Fahrenheit, who created a temperature scale based around the boiling and freezing points of water. He placed the boiling and freezing points of water 180 degrees apart, in part because of the idea that 180 degrees on a circle indicates two opposite points on the circle. The first wall thermometers were large cylinders that contained different alcohol solutions in small glass bubbles. Each solution corresponded to a specific temperature, and at that temperature the glass bubble would float in the middle of the tube. At temperatures below the one indicated by the bubble, that bubble would be sunk on the bottom of the tube. At temperatures higher, the bubble would float all the way to the top.

It took a great deal of time and effort to create each of these thermometers, since each alcohol concentration had to be measured very precisely for the thermometer to be accurate. The water also had to be filtered, since any impurities would make the buoyancy properties of the water different form the idealized standard.

Eventually a successor was found to these first wall thermometers in mercury. It was a rare metal to begin with, in that it was a metallic element, but it was found in liquid form on earth. Mercury is also toxic to humans, and so must be handled carefully. However, for the purposes of reading temperature, one feature of mercury stands out above the other oddities. It expands at a constant rate through a normal temperature range of 0-300 degrees Fahrenheit. This means that if mercury was placed in a vacuum sealed tube, it would expand to fill the tube as the temperature rose. Thus, the first single substance wall thermometers were born. Simply marking the different temperatures on a piece of metal or wood behind the mercury vial could show the proper temperature.

Wall thermometers today have progressed to more sensitive; and less volatile substances that mercury. The health hazard alone makes mercury unsuitable for use in a breakable glass vial. A metal coil can now be used to show temperature in a radial, rather than linear fashion. Most metals, like mercury, expand at a constant rate through the normal range of earth temperatures. This means that a metal coil that is heated will exert increasing pressure on the ends of the coil if it is heated. This can be used to make a thermometer that reads like a clock with one hand. Digital thermometers take the guesswork out of reading and simply display numerals that indicate the current temperature.

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