THERMOMETERS:

Definition: A device to measure temperature.

Liquid-in-glass

This is the most commonly used thermometer to measure air temperature. These thermometers are inexpensive to make and easy to use. These thermometers have a glass bulb attached to a sealed glass tube. A very thin opening, called a bore, extends from the bulb down through the center of the tube. The bulb is typically filled with either mercury or red-colored alcohol and the liquid is free to move from the bulb into the tube. When air temperature increases, the liquid in the bulb expands, forcing it to rise up the tube. When air temperature decreases, the liquid cools and contracts, drawing it out of the tube and back into the bulb.

When temperature is measured with a common liquid in glass thermometer, it is important to keep the instrument shaded. It is impossible to measure air temperature accurately in direct sunlight because the thermometer absorbs radiation from the sun in addition to the kinetic energy being transferred from the air molecules in contact with the glass. Energy transferred to the liquid from both of these sources will result in a temperature reading which is greater than the actual air temperature.

EXPLORES! ASKS?

Why does the temperature on the sign at the bank always seem to be higher than the actual temperature reported on the radio or TV?

Electrical

This type of thermometer is used for highly accurate temperature measurements. One type of electrical thermometer is an electrical resistance thermometer. This thermometer does not actually measure temperature, but resistance in a platinum or nickel wire, whose resistance changes as temperature changes. An electrical meter measures the changes in resistance which is related to temperature using a technique called calibration. In order to use calibration, you must determine in advance exactly how the resistance of the metal varies with changes in temperature.

A second type is called a thermistor. Thermistors are electrical resistance thermometers which use ceramic materials whose resistance increases as temperature increases. The thermistor is used on radiosondes which are attached to balloons to measure temperature in the upper atmosphere.

A third type is called a thermocouple. The thermocouple works on the principle that a metal will react to temperature changes by affecting the amount of electrical current which flows through it. If two different metals are connected, then their response to temperature changes will be different. By measuring the change in an electrical current which passes across these two metals, the actual temperature can be determined using calibration.

Radiometers

This type of thermometer actually measures the amount of emitted radiation (usually infrared) and the wavelength of maximum emission of a particular substance. With calibration techniques, the amount of radiation and the wavelength of maximum emission are directly related to the temperature of that substance. This type of sensor is placed on orbiting meteorological satellites to detect surface temperature (land or sea) if the sky is clear and cloud-top temperature if the sky is cloudy.

Bimetallic

This type of thermometer consists of two different metals (usually iron and brass) which are welded together to form a single strip. As the temperature changes, one metal will expand more than the other, causing the strip to bend. This bending is amplified by a series of levers which is attached to a pointer on a scale. Your household thermostat is an example of a bimetallic thermometer. A thermograph is another type of bimetallic thermometer. The metal strip is connected to a pen which traces the temperature on a piece of paper attached to a drum which rotates with time. Bimetallic thermometers are not nearly as accurate as the thermometers discussed above.

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