What does "capillary" mean here? "The typical laboratory thermometer contains a bulb (reservoir) of temperature-sensing liquid at the bottom; it is this portion of the thermometer that actually senses the temperature. The glass barrel of the thermometer above the liquid bulb contains a fine capillary opening in its center, into which the liquid rises as it expands in volume when heated. The capillary tube in the barrel of the thermometer has been manufactured to very strict tolerances, and it is very regular in cross-section along its length. This ensures that the rise in the level of liquid in the capillary tube as the thermometer is heated will be directly related to the temperature of the thermometer's surroundings. "