Temperature provides a quantification of the hotness or coldness of a substance, which is a direct reflection of the average kinetic energy of its constituent particles. Atoms and molecules are constantly in motion; a higher temperature signifies greater average kinetic energy and faster movement, while a lower temperature means slower movement. Thermometers are specifically designed instruments that translate this energetic motion into a readable, standardized numerical value.
The Physical Laws Governing Measurement
Every contact thermometer relies on the fundamental concept of thermal equilibrium to function. When a thermometer is placed in contact with an object, heat energy transfers between the two until they reach the same temperature. At this point, the net flow of heat ceases, and they are said to be in thermal equilibrium. The thermometer, having reached the object’s temperature, is then ready to display the measurement.
Thermometers rely on “indicator properties,” which are predictable physical characteristics of a substance that change reliably with temperature. For example, the volume of a liquid, the electrical resistance of a metal, or the length of a metal strip are all indicator properties that can be measured to infer temperature. By measuring the change in one of these properties, the thermometer can calculate and display the corresponding temperature.
Measurement via Thermal Expansion
The liquid-in-glass thermometer operates on the principle of thermal expansion. When a substance absorbs heat, its particles vibrate more vigorously and move farther apart, causing the material to increase in volume or length. A traditional thermometer contains a liquid, such as alcohol or mercury, within a reservoir bulb and a thin capillary tube.
As the temperature rises, the liquid expands. Because the glass tube has a significantly smaller volume than the liquid’s change in volume, the liquid is forced to rise visibly up the narrow column. The height of the liquid column is directly proportional to the volume change and is marked against a calibrated scale to provide a temperature reading. Similarly, a bimetallic strip thermometer uses two different metals, such as brass and steel, bonded together. Since each metal expands at a different rate when heated, the strip is forced to bend or coil, and this mechanical movement is used to turn a pointer on a dial.
Measurement via Electrical Resistance
Modern digital thermometers often employ materials that exhibit a predictable change in electrical resistance with temperature, such as Resistance Temperature Detectors (RTDs) and thermistors. RTDs are typically made of pure metal wire, often platinum, whose electrical resistance increases linearly as temperature rises. Increased thermal energy causes the atoms to vibrate more, impeding the flow of electrons and increasing resistance.
Thermistors are typically made from semiconductor materials and can exhibit either a positive or negative temperature coefficient, meaning their resistance can increase or decrease significantly with temperature. Digital circuitry within the thermometer passes a constant current through the sensor and measures the resulting voltage drop. This voltage is converted into a resistance value, which a microprocessor translates using a pre-programmed formula into the final temperature displayed on the screen.
Measurement via Infrared Radiation
Non-contact thermometers, such as those used for measuring ear or forehead temperature, do not require thermal equilibrium. All objects with a temperature above absolute zero emit electromagnetic radiation in the infrared spectrum. The intensity of this emitted infrared energy is directly related to the object’s absolute temperature, a relationship described by the Stefan-Boltzmann law.
An infrared thermometer uses a lens system to focus this thermal radiation onto a specialized sensor, often a thermopile. The thermopile is an array of tiny thermocouples that absorb the infrared energy and convert it into a small electrical signal (voltage). The device’s electronics analyze the strength of this voltage, using the law of radiation to calculate the object’s surface temperature from a distance.
Establishing Temperature Scales
The raw physical changes measured by a thermometer must be translated into standardized numbers that are consistent and universally understood. This is accomplished by establishing a temperature scale using specific, reproducible physical phenomena called fixed points. Historically, the freezing and boiling points of pure water at standard atmospheric pressure were chosen to calibrate the Celsius and Fahrenheit scales.
For the Celsius scale, the freezing point of water is defined as \(0^circtext{C}\) and the boiling point as \(100^circtext{C}\), creating 100 divisions. The Fahrenheit scale uses \(32^circtext{F}\) for the freezing point and \(212^circtext{F}\) for the boiling point, resulting in 180 divisions. Modern scientific scales, such as the International Temperature Scale of 1990 (ITS-90), use more precise reference points, including the triple point of water—the single temperature and pressure where water exists simultaneously as a solid, liquid, and gas. These fixed points provide anchors that allow for the accurate calibration of thermometers and ensure that a temperature reading is consistent.