AN APPARATUS FOR REALIZING THE TRIPLE POINT OF MERCURY

A mercury triple point apparatus has been developed which permits the observation of long freezes and melts with little intervention. We hav e developed a sealed glass cell containing 2,5 kg of mercury which was filled simply by pouring rather than by vaccum distillation. The cell is installed in a temperature-controlled bath of our own design. Melt ing plateaux of more than 24 hours duration are recorded with the bath setpoint approximately 3-degrees-C higher than the mercury triple poi nt temperature. Much longer plateaux are possible by setting the bath closer to the triple point temperature. The temperature of the triple point of mercury has been determined as 234,3085 K on the IPTS-68 with a standard deviation of 0,1 mK based on measurements with five platin um resistance thermometers. The result is in excellent agreement with the values 234,3083 K and 234,3086 K as determined at the NIST (former ly the NBS, USA) by Furukawa and Bigge and at the NPL (UK) by Chattle and Butler, respectively. A platinum resistance thermometer tracks the hydrostatic gradient within the cell over a distance of 10 cm and is in agreement with the value of 7,1 mK per metre of liquid as given in the texts of the IPTS-68 and the ITS-90. The measurements yield a valu e of 234,3159 K on the ITS-90 for the triple point of mercury cell inv estigated.