Features of a common thermometer

Bulb – Carries the liquid in the thermometer. It has a thin glass wall for effective heat transmission between the liquid and body whose temperature is taken.

Capillary bore – Liquid expands and contracts along the capillary tube. It is
narrow for high degree of accuracy.

Glass stem – Is a thick wall surrounding the capillary bore. It also serves as a magnifying glass for easy reading of scale.

For a clinical thermometer, it has a narrow constriction just above the the bulb to prevent the mercury level from falling down when it is in contact
with the human body.