The image of a bulb thermometer with Celsius (centigrade) temperature scale on one side and Fahrenheit scale on the side. A capillary tube passes through the center, ending in a bulb at the bottom. The bulb contains the reservoir of liquid. The colored liquid moves up and down the capillary tube as the temperature changes. The scale indicates the temperature according to how far up or down the capillary tube the liquid has moved. The volume of a tube is calculated using the formula Volume is equal to pi multiplied with the radius squared and the length. For a given volume, if the radius of a tube is decreased (as in a capillary tube), the length of the liquid column increases. Any change in volume is then easier to see. The image of a wide tube with a compressed scale is shown. Liquid rises less in a wide tube for the same temperature change. The image of a narrow tube with an expanded easy-to-read scale is shown. Liquid rises more in a narrow tube for the same temperature change.