Until the discovery of true thermodynamic temperature, the mercury thermometer usually defined the temperature.
These issues were explored experimentally with the gas thermometer.
In practice, these variations were very slight and remained close to the thermodynamic temperature, once the latter was discovered. Later thermometers that used a liquid other than mercury also gave slightly different temperature readings. These points are adequate for approximate calibration, but both the freezing and boiling points of water vary with atmospheric pressure.
Divide the length between the two marks into 100 equal parts. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour. This point is the freeze/thaw point of water. Place the cylinder of the thermometer in melting ice made of pure water and mark the point where the fluid in the thermometer stabilises. He named it centigrade (100 steps).įinally, Celsius proposed a method of calibrating a thermometer: One year later, Frenchman Jean-Pierre Christin proposed to invert the scale with the freezing point at 0 ☌ (32 ☏) and the boiling point at 100 ☌ (212 ☏). he chose to set the boiling point of pure water at 0 ☌ (212 ☏) and the freezing point at 100 ☌ (32 ☏). When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down", i.e. This was related to the rapid cooling (and contraction) of the glass. At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly. He found a similar fixed point in the calibration of boiling water to water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure Celsius noted this). By performing the same experiment over and over again, he discovered that ice always melted at the same calibration mark on the thermometer. The experiments for reaching a good calibration of his thermometer lasted for 2 winters. The distinction of Celsius was to use the condition of melting and not that of freezing. This was not a new idea, since Isaac Newton was already working on something similar. To define his scale Celsius used two fixed temperature points: the temperature of melting ice and the temperature of boiling water, both under atmospheric pressure of the standard atmosphere. The thermometer was used by the originators of the Fahrenheit and Celsius scales.Īnders Celsius, a Swedish scientist, devised the Celsius scale, which was described in his publication The origin of the Celsius temperature scale in 1742. 
See also: Liquid-in-glass thermometer and Timeline of temperature and pressure measurement technology In principle, thermometers made of different material (e.g., coloured alcohol thermometers) might be expected to give different intermediate readings due to different expansion properties in practice the substances used are chosen to have reasonably linear expansion characteristics as a function of thermodynamic temperature, and so give similar results. 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. The space above the mercury may be filled with nitrogen gas or it may be at less than atmospheric pressure, a partial vacuum. The volume of mercury changes slightly with temperature the small change in volume drives the narrow mercury column a relatively long way up the tube.
It consists of a bulb containing mercury attached to a glass tube of narrow diameter the volume of mercury in the tube is much less than the volume in the bulb. The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam (1714). Mercury-in-glass thermometer for measurement of room temperature.
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