Mercury-in-glass thermometer related to the rapid cooling (and contraction) of the glass. When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down", i.e. he chose to set the boiling point of pure water at 0 °C (212 °F) and the freezing point at -100 °C (-32 °F). One year later, Frenchman Jean-Pierre Christin proposed to invert the scale with the freezing point at and the boiling point at . He named it Centigrade (100 grades). Finally, Celsius proposed a method of calibrating a thermometer: These points are adequate for approximate calibration but both vary with atmospheric pressure.
Beckmann thermometer inverted and gently tapped so that the mercury in the reservoir lodges in the bend (B) at the end of the stem. Next, the bulb is heated until the mercury in the stem joins the mercury in the reservoir. The thermometer is then placed in a bath one or two degrees above the upper limit of temperatures to be measured. The upper end of the tube is gently tapped with the finger, and the mercury suspended in the upper part of the reservoir will be jarred down, thus separating it from the thread at the bend (B). The thermometer will