3. Thermometers: Types and Principles

A thermometer is an instrument used to measure temperature. All thermometers work on the principle that some measurable physical property of a substance changes in a regular, predictable way with temperature (this is called a thermometric property) — for example, the volume of a liquid, the electrical resistance of a metal, the voltage generated across a junction of two metals, or the intensity and colour of radiation emitted by a hot object.

3.1 Liquid-in-Glass (Mercury) Thermometer

This is the traditional and most familiar type of thermometer, consisting of a narrow glass capillary tube connected to a bulb containing mercury. It works on the principle of thermal expansion: as temperature rises, the mercury in the bulb expands and rises up the narrow, uniformly bored capillary tube, and the temperature is read off a calibrated scale printed alongside the tube. Mercury is preferred over water for several reasons: it does not wet the sides of the glass tube, it is opaque and easily visible, it has a high boiling point (357°C) and low freezing point (−39°C) giving it a wide usable range, it expands almost uniformly with temperature, and it is a good conductor of heat so it responds quickly. Clinical (medical) thermometers historically used mercury with a special constriction near the bulb that prevents the mercury column from falling back on its own once removed from the body, allowing the peak reading to be recorded; these have largely been phased out in favour of digital thermometers due to the toxicity of mercury. Alcohol (often dyed red) thermometers are used for measuring very low temperatures, such as in cold regions or laboratory freezers, since alcohol has a much lower freezing point (−115°C) than mercury, though it has a lower boiling point (78°C), limiting its use at high temperatures.

3.2 Digital Thermometer

Digital thermometers commonly use a thermistor — a semiconductor device whose electrical resistance changes very sharply and predictably with temperature. A small electric current is passed through the thermistor, its resistance is measured electronically, and a microprocessor converts this resistance value into a temperature reading displayed on an LCD or LED screen. Digital thermometers are widely used today in clinical settings (oral, rectal, and infrared forehead/ear thermometers) because they are safer (no mercury), faster, and more precise than liquid-in-glass types.

3.3 Thermocouple Thermometer

A thermocouple consists of two dissimilar metal wires (common pairs include copper–constantan, chromel–alumel, or iron–constantan) joined together at one end (the 'hot' or measuring junction) while the other ends (the 'cold' or reference junction) are kept at a known, stable reference temperature. This works on the Seebeck effect: when the two junctions are at different temperatures, a small electromotive force (EMF), typically in the millivolt range, is generated in the circuit, and the magnitude of this EMF is proportional to the temperature difference between the junctions. By measuring this EMF, the temperature at the hot junction can be calculated. Thermocouples are extremely useful because they are rugged, respond quickly, can measure a very wide range of temperatures (from about −200°C to over 1600°C depending on the metal pair used), and can be made very small for use in confined spaces such as furnaces, engines, and industrial process equipment.

3.4 Pyrometer

A pyrometer is a non-contact thermometer used for measuring very high temperatures — such as those of furnaces, molten metals, and other extremely hot objects — where a physical probe would itself melt or where contact is otherwise impractical. It works on the principle that every hot object emits thermal (infrared/optical) radiation, and the intensity and spectral distribution of this radiation depend on the object's temperature (this relates to the concept of black-body radiation). An optical pyrometer typically compares the brightness/colour of the radiation from the hot object with that of a calibrated reference filament, adjusting the filament's current until the two appear identical in brightness; the current required gives a measure of temperature. Radiation pyrometers focus the incoming thermal radiation onto a detector and directly convert the radiant intensity into a temperature reading. Pyrometers are widely used in steel plants, glass factories, foundries, and for measuring the temperature of the sun's surface.

3.5 Comparison of Common Thermometers