Which type of thermometers are recognized as the oldest and simplest for measuring temperature?

Liquid-in-glass thermometers are considered the oldest and simplest type of thermometer used to measure temperature. They operate on the principle of thermal expansion of liquids, typically mercury or colored alcohol, which expands and contracts as the temperature changes. As the temperature increases, the liquid expands and rises in the glass tube, and as the temperature decreases, it contracts and falls. This direct and visible response makes liquid-in-glass thermometers straightforward to read and understand, making them a foundational tool in temperature measurement.

In contrast, digital thermometers utilize electronic sensors to measure temperature and provide a digital output, which involves more complex technology. Bimetallic thermometers use two different metals bonded together that expand at different rates, creating a mechanical movement to indicate temperature, which adds additional components and complexity. Infrared thermometers, while convenient for non-contact temperature measurement, rely on detecting infrared radiation and require more sophisticated sensors and calibration methods. Each of these alternatives showcases advancements in technology, but the simplicity and effectiveness of liquid-in-glass thermometers mark them as the oldest and most accessible type among temperature measuring devices.