On the discharge between concentric cylinders in gases at low pressures

In some previous communications the author has described investigations upon the length of the Crookes dark space and the relation between current and potential in the discharge between large plane aluminium electrodes in different gases at various pressures. For all the chemically active gases, and also for the gases of the helium group when these are not in a state of great purity, the two following equations were found to hold over a considerable range of the four variables:— D = A/P + B/√C, (I) V = E + F√C/P. (II) where D is the length of the dark space, P the pressure, V the potential difference between the electrodes, and C the current density. On examining the values of the four constants A, B, E, and F, it will be seen that although A, E, and F have widely different values for different gases the variations in the values of B (taking the cases of gases in which it was fairly constant) are very small and may be regarded as insignificant considering the difficulty of its accurate determination.