Structure of a Mercury Clinical Thermometer: Bulb: One end of the thermometer contains a bulb filled with mercury. Capillary Tube: Attached to the bulb is a narrow capillary tube, which is marked with calibrated scale. The mercury expands up this tube when heated and contracts when cooled. Scale: The thermometer has a scale, either in Celsius (°C) or Fahrenheit (°F), or sometimes both, which allows the user to read the body temperature. Constriction: Many clinical mercury thermometers have a constriction in the capillary tube just above the bulb. This constriction holds the mercury in the tube after it has expanded and prevents it from returning immediately to the bulb once it’s removed from a person’s mouth. This makes it easier to read the temperature. Usage: Before Use: The thermometer is shaken down to ensure the mercury is in the bulb and not in the stem. Position: It is then placed under the tongue (oral), in the armpit (axillary), or in the rectum (rectal), depending on the method of measurement. Reading: After a few minutes, the thermometer is removed, and the temperature is read by noting the level of the mercury column. After Use: The thermometer is shaken down again to reset it for the next use and is then cleaned and disinfected.