300 years ago, Daniel Gabriel Fahrenheit, a physicist, engineer and glass blower invented the mercury-in-glass thermometer. The mercury thermometer consists of a bulb containing mercury attached to a narrow glass tube, the volume of the mercury is much less than the volume of the bulb. The volume of mercury changes slightly when placed in different temperatures, the small change in volume causes the mercury to travel a relatively long way up the tube. The space above the mercury may be filled with nitrogen or it may be at less than atmospheric pressure, a partial vacuum. In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium with a temperature standard such as an ice/water mixture, and then with another standard such as water/vapour, and the tube is divided into regular intervals between the fixed points.
Fundamentally, thermometers made of different materials (e.g., coloured alcohol thermometers) might be expected to give different average readings due to different expansion properties. Daniel Gabriel Fahrenheit also developed the temperature scale now named after him; farenheit.
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