A glass thermometer is a sealed glass tube containing a temperature-sensitive liquid, such as dyed alcohol or a silver-colored liquid like mercury or galinstan. This liquid expands or contracts predictably with temperature changes. The device measures temperature based on thermal expansion. Correctly interpreting the reading requires proper preparation and careful observation of the liquid column’s height against the calibrated scale. Accurate measurement depends entirely on following a precise technique.
Preparing the Thermometer for Measurement
Before taking any measurement, the glass thermometer must be properly prepared. First, clean the device with cool water and a mild disinfectant, especially for clinical thermometers, to prevent contamination. Avoid handling the bulb, as hand heat can cause the liquid to expand and skew the reading.
A crucial preparatory action is to “shake down” the liquid column, which resets the thermometer by forcing the liquid back into the reservoir bulb. Hold the end opposite the bulb securely and use a quick, sharp wrist movement to swing the thermometer downward. The goal is to bring the liquid level well below the expected temperature, typically around 95°F or 35°C, ensuring an accurate baseline. If the column is not lowered, the thermometer will only register temperatures higher than the current reading.
Deciphering the Measurement Scale
Understanding the markings etched onto the glass stem is fundamental to accurate reading. The scale is calibrated in either Fahrenheit (°F) or Celsius (°C), sometimes both, with the unit clearly printed on the glass. The liquid inside is visible as a thin, continuous column, which may be silver or a bright color like red or blue.
The scale is composed of major and minor lines representing different temperature increments. Long, numbered lines indicate whole degrees, such as 98, 99, and 100 degrees Fahrenheit. Shorter lines, known as graduations, represent fractions of a degree. For example, a common Fahrenheit clinical thermometer has four short lines between each whole number, meaning each small line represents 0.2°F. A typical Celsius scale often has nine short marks between whole degrees, with each mark representing 0.1°C.
Technique for Accurate Reading
After the thermometer has equilibrated with the temperature being measured, the physical reading must be performed with precision. Hold the thermometer horizontally at eye level, ensuring your line of sight is perfectly perpendicular to the liquid column. This positioning eliminates parallax error, which is an observational error that occurs when a reading is taken from an angle, causing the liquid level to appear inaccurate.
Gently rotating the thermometer between your thumb and forefinger will reveal the liquid column, as the glass stem is often slightly flattened to act as a magnifier. Align the top of the liquid column with the corresponding scale markings. If the column rests directly on a major line, that whole number is the temperature. If it falls between lines, add the value of the nearest shorter line to the previous whole degree. Record the measurement immediately after noting the highest point of the liquid, as the temperature will begin to drop once the thermometer is removed from the heat source.