The cells were supplied by NPL.
The metal used for filling each cell had purity 99.9999%.
The sealed cells were fitted into an inconel tube with suitable insulators above it. In the insulators a hole was drilled through which a silica tube with a roughened outer surface was passed as a thermocouple guide.
The furnace used for metal freezing point was made by Carbolite––England, it was designed to operate until 1000 C and utilized double walled sealed sodium heat pipe, about 35 cm long to uniform the temperature around the fixed point cells and to obtain a short response time, adequate for automatic control of the temperature.
The furnace controller has a set point resolution of 0.1 C and is controlled by a computer through an RS232 interface.
For the calibration at the melting point of gold, the Pt extension coil was removed and a 2 cm length, 0.2 mm diameter of high purity gold wire (purity 99.99%) was welded at the hot junction of the thermocouple.
The thermocouple was placed in a uniform vertical furnace supplied by Johnson Matthy.
The temperature of the furnace was increased at the rate of 0.5 C/min.
At the melting point of Au the temperature remained stationary for at least 1.5 min before the thermocouple open circuits.