What is the purpose of the bulb at the base of a mercury-in-glass thermometer?

The purpose of the bulb at the base of a mercury-in-glass thermometer is to enhance sensitivity by containing most of the mercury. This design allows for a larger volume of mercury to expand and contract with temperature changes, resulting in a more noticeable movement of the mercury column in the narrow tube above.

When the temperature increases, the mercury expands into the capillary tube, allowing it to rise and indicate the temperature on the scale marked on the thermometer. Conversely, when the temperature decreases, the mercury contracts and returns to the bulb. The larger amount of mercury in the bulb improves the thermometer's response to temperature changes, making it more sensitive and providing a more accurate reading.

While other choices mention aspects like size, pressure relief, and measuring specific points, they do not pertain directly to the fundamental function that the bulb serves in enhancing the thermometer's sensitivity and responsiveness to temperature changes.