How Does a Thermometer Work? Measuring Something You Cannot Touch

Heat and temperature are frequently confused and are different quantities. Heat is energy in transit between bodies at different temperatures, measured in joules. Temperature is a measure of the average kinetic energy of the particles in a substance, and its practical meaning is the direction heat will flow: from higher to lower, always. A bathtub of warm water contains far more heat energy than a spark, and the spark has a much higher temperature. That definition explains why a thermometer works at all: placed in contact with something, it exchanges energy until both reach the same temperature, which is thermal equilibrium, and the thermometer then reports its own state. It also explains the main source of error in practice, since the thermometer must be small enough not to change what it is measuring, and must be given time to equilibrate, which is why a reading taken too early is always wrong in the direction the instrument came from.

Each design exploits a different temperature-dependent property, and the choice follows from range, speed and accuracy:

• •Liquid in glass, relying on thermal expansion, cheap and requiring no power; mercury has an almost linear expansion and a wide liquid range and is being phased out for toxicity, with dyed alcohol used instead at a cost of a lower upper limit
• •Bimetallic strip, two bonded metals with different expansion rates that bend with temperature, used in dial thermometers, ovens and older thermostats, robust and not very precise
• •Thermocouple, exploiting the voltage generated at a junction between two dissimilar metals, which covers an enormous range including very high temperatures, is cheap and rugged, and is less accurate than the alternatives
• •Resistance thermometer, usually platinum, whose electrical resistance rises predictably with temperature, giving high accuracy and stability, which makes it the workhorse of industrial and laboratory measurement
• •Thermistor, a semiconductor whose resistance changes sharply with temperature, very sensitive over a narrow range, which is what most digital clinical thermometers contain
• •Infrared thermometer, measuring the thermal radiation a surface emits, which requires no contact and reads only the surface, and whose accuracy depends on knowing how efficiently that surface radiates

Three scales are in general use and they are not interchangeable in the way they are usually treated. Fahrenheit, devised in 1724, set its zero at the temperature of a particular freezing brine and produced a scale whose ordinary numbers happen to span the range of human weather comfortably. Celsius, from 1742, was defined by the freezing and boiling points of water at standard pressure, originally with the scale running the other way. Kelvin is the scientific scale and differs in kind, because it is absolute: its zero is the point at which the thermal motion of particles is at the minimum permitted, so there are no negative values, and a doubling of the kelvin value means a genuine doubling of thermal energy in a way that a doubling of Celsius does not. A degree kelvin and a degree Celsius are the same size, which is why conversion is a simple offset. Since 2019 the kelvin has been defined by fixing the value of the Boltzmann constant, which ties temperature to fundamental constants rather than to the properties of water.

Human body temperature is not a single number, and the everyday figure of 37 degrees Celsius comes from a study published in 1868 that measured axillary temperatures in a large German sample. Later work has consistently found a lower average, closer to 36.6, and analyses of long-run data suggest a genuine decline over the past century as well as differences in measurement. Temperature varies by site, with rectal, oral, axillary, tympanic and temporal artery readings differing systematically by a degree or more, so a measurement is meaningless without knowing where it was taken. It varies through the day by around half a degree on a circadian cycle, with a minimum in the early morning, and it varies with activity, menstrual cycle, age and ambient conditions. Infrared forehead scanners, which became ubiquitous during the pandemic, measure skin rather than core temperature and are strongly affected by sweat, cosmetics, ambient temperature and distance, which is why their sensitivity for detecting fever in screening settings has been reported as poor.

A thermometer measures temperature indirectly through something that varies with it: a liquid's expansion, a metal's resistance, a junction voltage or emitted infrared. It reports its own temperature after reaching equilibrium, which is why it must be small and given time. Temperature measures average particle kinetic energy and heat is energy in transit, which is why a spark is hotter than a bath containing far more heat. Kelvin is absolute and now defined by fixing the Boltzmann constant.