With the glass tube on the horizontal position as described in the
previous section, the pressure was first established at 20e26 mbar.
The filament voltage was fixed to a maximum of 7.00 V and its
current was read. The pressure was increased step by step, up to
ambient pressure (approximately 1000 mbar). At each step, the
potential difference generated in the thermocouple by the heating
of the filament was recorded. To be accurate, the reading was made
only when thermocouple voltage stabilized after each step variation
in pressure. That finished, the same procedure was repeated with
lower filament voltage down eventually to a value of 1.00 V. The
maximum filament current reached about 1.3 A. Ambient temperature
Tawas not the same for all curves, and varied between 25 and
27 C