Kinds of Thermometers

Liquid-in-glass Thermometers

The most common type of thermometer is the liquid-in-glass thermometer. It consists of a narrow, sealed glass tube with a bulb filled with mercury, alcohol (usually dyed red, to make it easily visible), or some other liquid at its lower end. As the temperature rises, the liquid expands and rises in the tube. A drop in temperature causes the liquid to contract, so that its level in the tube falls.

Bimetal Thermometers

A bimetal thermometer contains a strip consisting of two fused strips of different metallic substances (metals or alloys). The strip is usually in the form of a spiral or rod fixed at one end. When the temperature changes, one of the metallic substances expands or contracts more than the other, causing the strip to wind or unwind, or to bend. A pointer attached to the free end of the strip indicates the temperature. Bimetal strips are widely used in dial thermometers and in thermostats. Another type of thermometer using dissimilar metals is the thermocouple.

The thermograph, or recording thermometer, which provides in chart form a continuous record of temperature during a desired period of time, is often of the bimetal type. A pen or stylus attached to the free end of the bimetal strip records the temperature on a chart fastened to a rotating disk or drum.

Resistance Thermometers

determine temperature by measuring the change in electrical resistance of a metal wire or of a thermistor, a resistor composed of a semiconducting material. The wire or thermistor is usually enclosed in a slender rod, or probe. The temperature is indicated on a meter or as a digital display.

Infrared Thermometers

measure the infrared (heat) radiation given off by an object to determine its temperature. The pyroelectric thermometer contains a type of crystal that produces an electrical signal whose strength is proportional to the infrared radiation to which it is exposed. The most familiar use of such thermometers is in ear thermometers that measure infrared radiation from the tympanic membrane (ear drum) to determine body temperature. Other types of infrared thermometers include bolometers and pyrometers.

Gas Thermometers

The most common type, the constant-volume gas thermometer, determines temperature by measuring the pressure needed to keep the volume of a gas constant as the temperature varies. The pressure of a gas confined to a constant volume varies with temperature in a known manner; if the pressure of the gas is measured, its temperature can be easily calculated.

Liquid-crystal Thermometers

contain liquid crystals that change color over a certain range of temperatures. Various mixtures of liquid crystals are used, each mixture in a separate container and each becoming light in color at a specific temperature. In digital types, each container is covered with a stencil of a number that corresponds to the temperature at which the mixture in the container becomes light in color. Fever thermometers designed to be placed on the forehead and most indoor digital thermometers are of this type.