Thermometer

I

Introduction

Thermometer, instrument used to measure temperature. The most commonly used thermometer is the mercury-in-glass type, which consists of a uniform-diameter glass capillary that opens into a mercury-filled bulb at one end. The assembly is sealed to preserve a partial vacuum in the capillary. If the temperature increases, the mercury expands and rises in the capillary. The temperature may then be read on an adjacent scale. Mercury is widely used for measuring ordinary temperatures; alcohol, ether, and other liquids are also employed for this purpose.

The invention of the thermometer is attributed to Galileo, although the sealed thermometer did not come into existence until about 1650. The modern alcohol and mercury thermometers were invented by the German physicist Gabriel Fahrenheit, who also proposed the first widely adopted temperature scale, named after him, in which 32° F is the freezing point of water and 212° F is its boiling point at standard atmospheric pressure. Various temperature scales have been proposed since his time; in the centigrade, or Celsius, scale, devised by the Swedish astronomer Anders Celsius and used in most of the world, the freezing point is 0°, the boiling point is 100°.

II

Types of Thermometers

A wide variety of devices are employed as thermometers. The primary requirement is that one easily measured property, such as the length of the mercury column, should change markedly and predictably with changes in temperature. The variation of that property should also remain fairly linear with variations in temperature. In other words, a unit change in temperature should lead to a unit change in the property to be measured at all points of the scale.

The electrical resistance of conductors and semiconductors increases with an increase in temperature. This phenomenon is the basis of the resistance thermometer in which a constant voltage, or electric potential, is applied across the thermistor, or sensing element. For a thermistor of a given composition, the measurement of a specific temperature will induce a specific resistance across the thermistor. This resistance can be measured by a galvanometer (see Electric Meters) and becomes a measure of the temperature.

---

---

Various thermistors made of oxides of nickel, manganese, or cobalt are used to sense temperatures between -46° and 150° C (between -50° and 300° F). Similarly, thermistors employing other metals or alloys are designed for use at higher temperatures; platinum, for example, can be used up to 930° C (1700° F). With proper circuitry, the current reading can be converted to a direct digital display of the temperature.

Very accurate temperature measurements can be made with thermocouples (see Thermoelectricity), in which a small voltage difference (measured in millivolts) arises when two wires of dissimilar metals are joined to form a loop, and the two junctions have different temperatures. To increase the voltage signal, several thermocouples may be connected in series to form a thermopile. Since the voltage depends on the difference of the junction temperatures, one junction must be maintained at a known temperature; otherwise an electronic compensation circuit must be built into the device to measure the actual temperature of the sensor.

Thermistors and thermocouples often have sensing units less than ‚ cm (less than s in) in length, which permits them to respond rapidly to temperature changes and also makes them ideal for many biological and engineering uses.

The optical pyrometer is used to measure temperatures of solid objects at temperatures above 700° C (about 1300° F), where most other thermometers would melt. At such high temperatures, solid objects radiate sufficient energy in the visual range to permit optical measurement by exploiting the so-called glow color phenomenon. The color at which hot objects glow changes from dull red through yellow to nearly white at about 1300° C (about 2400° F). The pyrometer contains a light bulb type of filament controlled by a rheostat (dimmer switch) that is calibrated so that the colors at which the filament glows correspond to specific temperatures. The temperature of a glowing object can be measured by viewing the object through the pyrometer and adjusting the rheostat until the filament blends into the image of the object. At this point the temperatures of the filament and the object are equal and can be read from the calibrated rheostat.

Another temperature-measuring device, used mainly in thermostats, relies on the differential thermal expansion between two strips or disks made of different metals and either joined at the ends or bonded together.

III

Special-Purpose Thermometers

Thermometers may also be designed to register the maximum or minimum temperature attained. A mercury-in-glass clinical thermometer, for example, is a maximum-reading instrument in which a trap in the capillary tube between the bulb and the bottom of the capillary permits the mercury to expand with increasing temperature, but prevents it from flowing back unless it is forced back by vigorous shaking. Maximum temperatures reached during the operation of tools and machines may also be estimated by special paint patches that change color when certain temperatures are reached.

IV

Accuracy of Measurement

Accurate measurement of temperature depends on the establishment of thermal equilibrium between the thermometric device and its surroundings; that is, when at equilibrium no heat is exchanged between the thermometer and the material it touches or material in its vicinity. A clinical thermometer, therefore, must be inserted long enough (more than one minute) to reach near-equilibrium with the human body to yield an accurate reading. It should also be inserted deep enough, and have sufficient contact with the body, to indicate temperature accurately. These conditions are almost impossible to achieve with an oral thermometer, which generally indicates a body temperature lower than that given by a rectal thermometer. Insertion times can be significantly reduced with small, rapidly reacting thermometers such as thermistor devices.

Any thermometer indicates only its own temperature, which may not agree with the actual temperature of the object to be measured. In measuring the air temperature outside a building, for example, if one thermometer is placed in the shade and one in the sun, only a few centimeters away, the readings on the two instruments may be quite different, although the air temperature is the same. The thermometer in the shade may lose heat by radiation to cold building walls. Its reading, therefore, will be slightly below the true air temperature. On the other hand, the thermometer placed in the sun will absorb the sun's radiant heat. As a result, the indicated temperature may be significantly above the true air temperature. To avoid such errors, accurate temperature determinations require the shielding of the thermometer from hot and cold sources to or from which heat might be transferred by radiation, conduction, or convection.

See Heat Transfer.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain