Calibrating a Thermometer Using Celsius's Two Fixed Points

In 1742, Anders Celsius — a Swedish astronomer at Uppsala University — presented a paper to the Royal Swedish Academy of Sciences proposing a temperature scale based on two reproducible reference points: the freezing point and boiling point of water. His key insight was that these phase transitions occur at fixed, universal temperatures under standard atmospheric pressure (760 mmHg / 101.325 kPa), making them ideal calibration anchors. Celsius's original scale was inverted from what we use today: he assigned 0° to the boiling point and 100° to the freezing point. The scale was reversed after his death (1744), likely by Carl Linnaeus or instrument maker Daniel Ekström. This blueprint recreates Celsius's original experimental method: establishing the two fixed points with a mercury-in-glass thermometer, verifying that boiling point varies with atmospheric pressure, and graduating the interval into 100 equal divisions. The procedure requires careful technique — the mercury must equilibrate fully at each reference point, and the barometer reading must confirm standard pressure for the boiling point to be valid. Celsius documented that a 27 mmHg pressure change shifts the boiling point by approximately 1°C, an observation still used in precision thermometry today.