Studies with Ionization Gauge. II. Relation between Ionization Current at Constant Pressure and Number of Electrons per Molecule

Calibration constants for the ionization gauge in various gases.—The relation for the pressure $P=K\times \mathrm{positive}$ ionization, where $K$ is a constant, was found to hold for Hg vapor (.025 to.21 bar) and for iodine vapor assuming published values of vapor pressures. For ${\mathrm{H}}_2$, He, Ne, ${\mathrm{N}}_2$ and CO at pressures around 50 bars, readings were compared with those of a McLeod gauge and the values of $K$ determined. For the same electrical conditions (anode voltage and electron current) $\frac{1}{K}$ is found to be proportional to the total number of electrons per molecule or molar number $N$, (except for ${\mathrm{H}}_2$ and He for which $\mathrm{KN}$ is half as large). Measuring $i_{+}\mathrm{in}$ microamps and $P$ in bars, the value of the constant $\mathrm{KN}$ is 10.4 for an electron current at 0.5 m-amp. and an accelerating voltage of 125 volts. Results for the vapor pressure of water and of Hg${\mathrm{I}}_2$ also agree with this relation.