More then just a simple, tried and true way to tell the temperature, thermometers can be ornamental too. Outdoor thermometers can be functional as well as decorative. The thermometers we are used to seeing are referred to as mercury in glass. The mercury in glass thermometer was invented by Dr. Daniel Fahrenheit. If that name sounds familiar, it should. Dr. Fahrenheit also developed the temperature scale of the same name.

The mercury in glass thermometer is defined as a primary thermometer, since there is no need for it to be calibrated against a known value. The thin glass tube or capillary contains a small amount of mercury in a reservoir referred to as a bulb. The mercury often has been tinted red to make the thermometer easier to read. The tube or capillary either is filled with an inert gas like nitrogen or is drawn to create a vacuum. As the ambient temperature rises, the mercury expands and rises in the tube.

The principle behind the mercury in glass thermometer is simple. As the ambient temperature rises, the mercury expands and rises from the bulb through the capillary. The scale of the thermometer is spaced accordingly. Even though Dr, Fahrenheit invented the mercury in bulb thermometer, the same system can be used to measure in the Celsius scale. The Celsius scale is the invention of Anders Celsius. Anders Celsius had originally thought of the thermal scale as an inverse of the Fahrenheit scale. The boiling point of water would be represented as zero and the freezing point of water would be represented by 100.

It was not until seventeen forty four that Celsius developed the final version of his Celsius scale. The freezing point of water, or thirty two degrees Fahrenheit, would be zero. The boiling point of water, or two hundred and twelve degrees Fahrenheit, would be represented by one hundred. The remaining length of the scale would be divided into increments of one hundred. Similar to the Celsius scale, the Centigrade scale is used generally to state temperature measurements in scientific terms.

Countries that have adopted the metric system of measurement use the Celsius scale to measure temperature. Countries like the United States still use the Fahrenheit scale for measuring atmospheric temperature. Many household thermometers display both temperature scales.

Mercury freezes at approximately thirty seven degrees Fahrenheit. The closer mercury comes to freezing, the less accurate the thermometer becomes. Unlike water, which expands as it freezes, mercury will not expand so there is no danger of the thermometer bulb breaking. The boiling point of mercury is so high that its use in a glass thermometer is perfectly safe.

When it comes to the safety of mercury, there are some important things to know. Mercury is the heaviest of all elements and is a toxic substance. You should avoid exposure to mercury, either by contact with the skin or inhaling vapors emitted by uncontained mercury. Most household thermometers contain a very small amount of mercury and breaking your outdoor thermometer does not generally pose a health hazard.

Even so, medical thermometers, the old medicine cabinet standby, are slowly being replaced by digital thermometers. The old medicine thermometer is also referred to as a maximum thermometer. The principle of the maximum thermometer is similar to a standard mercury thermometer with one significant difference.

The capillary tube is constricted directly above the bulb. As the temperature rises and the mercury expands up the tube, the constriction prevents the mercury from returning to the bulb. This allows the thermometer to be accurately read even though the ambient temperature has fallen. The mercury returns to the bulb only after the thermometer is shaken.

Decorative outdoor thermometers do not require this type of resetting. Once you hang one on your porch or patio, the mercury quietly does its job. Like metal and tin signs, outdoor thermometers have been used for many years as a marketing device. Companies wishing to advertise their goods and services would give away outdoor thermometers to retailers and shopkeepers. Shopkeepers would in turn hang them outside their shops. Like antique metal and tin signs, many of these advertising thermometers have gone on to become quite collectible.

Metal reproductions of classic outdoor thermometers are hotter than ever. From famous makers of agricultural equipment to thermometers that feature Hollywood film stars. In addition to classic metal reproductions, outdoor thermometers come in many other shapes and sizes. Artists and designers have come up with an almost endless variety of fun themes. Nautical decor designs are popular and the thermometer is often teamed up with a barometer and humidity gauge. Birdhouses, garden gnomes, weather vanes and many other designs can brighten your yard or patio. Decorative outdoor thermometers are a great way to accent your patio furniture.

Placing your outdoor thermometer in the right spot is very important. A thermometer exposed to direct sunlight will read higher than the actual ambient temperature. The glass metal or other materials will absorb heat from the sun, causing the mercury to rise beyond the actual air temperature. A good spot to place your outdoor thermometer is in the shade. This is true whether you have a mercury in glass thermometer or a dial model.

Outdoor dial thermometers operate on a different principle than the mercury type. Dial thermometers rely on a bimetal coil. Similar to a thermocouple, the metals expand and contract in reaction to changes in temperature. The dial provides designers with a different palette, offering very distinctive and decorative possibilities.