Temperature measures the average kinetic energy of particles within a substance, indicating how hot or cold an object is. Thermometers quantify this property by translating changes in material characteristics into a readable temperature value.
Fundamental Principles of Measurement
Thermometers operate on the principle that predictable changes occur in a material’s physical properties as its temperature fluctuates. Different substances respond distinctly to temperature variations, exhibiting changes such as expansion, alterations in electrical resistance, or radiation emission. Temperature measurement involves detecting these changes, which are consistently linked to the material’s thermal state. By calibrating these changes against a standard scale, thermometers provide a numerical temperature reading.
Measuring Temperature with Expansion
One common method for temperature measurement relies on thermal expansion, where substances expand when heated and contract when cooled. This change in volume or length is a direct response to particle motion. Two thermometer types utilize this property: liquid-in-glass and bimetallic strip thermometers.
Liquid-in-glass thermometers typically contain a liquid like alcohol or mercury sealed within a narrow glass tube with a calibrated scale. As temperature rises, the liquid expands, forcing the column to rise within the capillary tube, indicating a higher temperature. Conversely, a drop in temperature causes the liquid to contract, and its level falls, reflecting a lower temperature.
Bimetallic strip thermometers consist of two different metals bonded together, each possessing a unique rate of thermal expansion. When heated, one metal expands more than the other, causing the bonded strip to bend or coil. This mechanical deformation is directly proportional to the temperature change. This bending motion is linked to a pointer that moves across a calibrated dial, providing a visual temperature reading.
Measuring Temperature with Electrical Signals
Modern digital thermometers measure temperature by converting changes in electrical properties into signals. These signals are processed and displayed as numerical readings. Two examples are thermistors and thermocouples.
Thermistors are resistors whose electrical resistance changes predictably with temperature. Negative temperature coefficient (NTC) thermistors are commonly used, where their resistance decreases as temperature increases. A small electrical current is passed through the thermistor, and the resulting change in voltage, due to the altered resistance, is measured by a circuit. This voltage change is correlated to a specific temperature value, which is digitally displayed.
Thermocouples operate on the Seebeck effect, generating a small voltage when two dissimilar metals are joined at a junction and subjected to a temperature difference. When the measuring junction is heated, a voltage is produced, the magnitude of which depends on the types of metals used and the temperature difference between the measuring junction and a reference point. This generated voltage, typically in the microvolt to millivolt range, is then measured and translated into a temperature reading by an electronic instrument.
Measuring Temperature with Infrared Light
Infrared thermometers measure temperature without physical contact, relying on the principle that all objects above absolute zero emit infrared radiation, a form of electromagnetic energy. The intensity of this emitted radiation is directly related to the object’s surface temperature. Hotter objects emit more infrared energy.
These thermometers contain a lens that collects the infrared energy radiated by the target object. This collected energy is then focused onto a detector, often a thermopile, which converts the infrared radiation into an electrical signal. The strength of this electrical signal corresponds to the amount of infrared energy detected. An internal processing unit then converts this electrical signal into a temperature reading, which is displayed on the device. This non-contact method is particularly useful for measuring temperatures of distant, moving, or hazardous objects.