Determination of the upper fixed point
This is done by pushing the thermometer through a hole in a cork and placing inside a double-walled copper vessel called a hygrometer (Fig. 14.2). Water is steal boiled in the lower part of the hygrometer, thus keeping the bulb surrounded pure water vapor at atmospheric pressure. For reasons already mentioned, important that the bulb should not be allowed to dip into the boiling water. thermometer is adjusted so that the mercury thread is visible just above the to the cork. When the thread has remained steady for some minutes its level is macron the stem by a light scratch. The double walls reduce loss of heat and consed
cooling of the vapor surrounding the thermometer, while the manometer see the diagram gives warning should the pressure inside the hydrometer differ fr atmospheric pressure. If the barometric pressure at the time is not equal to – mmHg, then the true boiling-point for the prevailing pressure must be as-cert from a table giving the variation of boiling-point with pressure. Due allowance then made when marking the stem.
Actually the Celsius scale as we know it today was first suggested by Linnaeus in 1745. Celsius used a similar scale in which the ice point was marked 100° and the steam point 0°