Temperature measurement, or thermometry, is the process of quantifying the intensity of heat energy in a substance. All thermometers must rely on a predictable, measurable change in a physical property that correlates directly with temperature. This principle allows a device to translate an invisible energy level into a visible, standardized reading. From liquid-filled instruments to digital scanners, the core function remains the same: using a material that reacts consistently to heat to provide an accurate thermal snapshot.
The Principle of Thermal Expansion
The traditional liquid-in-glass thermometer operates on the fundamental physical principle of thermal expansion. When heat is absorbed, the kinetic energy of the liquid’s molecules increases, causing them to move faster and further apart. This increased molecular motion results in a corresponding increase in the substance’s overall volume.
The liquid is held in a reservoir bulb at the base of the instrument. As the liquid expands from the heat, it is forced upward into a sealed glass tube with an extremely fine, narrow bore, known as a capillary. The narrowness of the capillary tube acts as a magnifying mechanism, making a small volumetric change appear as a much larger, readable rise along the calibrated scale.
Mercury was once favored for its linear expansion across a wide temperature range, but it has largely been phased out due to its toxicity. Modern liquid thermometers typically use safer ethanol or other organic liquids, often dyed red or blue for visibility within the slender glass channel. The final temperature reading is a direct observation of the liquid’s physical height against the marked scale.
Measuring Temperature Through Electrical Resistance
Modern contact digital thermometers utilize a specialized sensor called a thermistor. The term thermistor is a portmanteau of “thermally sensitive resistor,” describing an electrical component whose resistance changes predictably with temperature. Most consumer digital devices employ a Negative Temperature Coefficient (NTC) thermistor, composed of sintered metal oxides like manganese, nickel, or cobalt.
The NTC property means that as the temperature increases, the electrical resistance of the sensor material decreases dramatically. When the thermistor is heated, more charge carriers are freed within the semiconductor, allowing current to flow more easily and reducing the resistance. This inverse relationship between heat and resistance is reliable and rapid.
An electronic circuit within the thermometer continuously passes a small, controlled current through the thermistor’s sensing tip. This circuit precisely measures the resistance value, which is then fed into a microprocessor. The microprocessor contains a pre-programmed formula, often based on the Steinhart–Hart equation, that accurately correlates the measured resistance to a specific temperature value. This calculated digital value is then converted into a numerical display.
Non-Contact Measurement Using Infrared Radiation
Another class of digital thermometers uses non-contact technology, measuring temperature by detecting the infrared (IR) radiation emitted by an object. All objects above absolute zero constantly emit electromagnetic energy in the infrared spectrum, known as blackbody radiation. The intensity of this emitted IR energy is directly proportional to the object’s surface temperature, meaning hotter objects radiate more energy.
A non-contact thermometer focuses this emitted thermal energy onto a specialized sensor, typically a thermopile. The thermopile is an array of tiny thermocouples connected in series, which operate based on the Seebeck effect. When the thermopile absorbs the focused infrared radiation, it creates a temperature difference between its hot junction (the absorber) and its cold junction (the sensor body).
This temperature difference generates a small, measurable electrical voltage signal that corresponds to the intensity of the incoming IR energy. To ensure accuracy, the device also contains a separate internal thermistor to measure the sensor’s temperature, allowing the microprocessor to compensate for internal fluctuations. The calculated voltage is then converted and displayed as the object’s surface temperature, enabling rapid readings without physical contact.