Sparking potentials of dry air, humid air and water vapour between concentric sphere-hemisphere electrodes

The sparking potentials of dry air, humid air and water vapour between a pair of concentric sphere-hemisphere electrodes are measured, for both voltage polarities of the inner electrode, over a wide range of gas pressures. In dry air, the measurements are conducted over nearly five orders of magnitude of gas pressure. In dry air and for gas pressure >100 torr, corona is observed to precede the breakdown of the gap when the inner sphere is made negative. On the other hand, the corona inception and the breakdown of the gap occurs at the same potential for the positive polarity of the inner electrode. The total secondary ionisation coefficient in dry air is evaluated from the measured sparking potential, and found to depend on the electrode polarity. The influence of the partial-water-vapour pressure on the sparking potential of air is measured. Practical mixtures of air and partial water vapour are employed. It is observed that the addition of water vapour increases the sparking potential for both the negative and the positive voltage polarities of the inner electrode. The relative increase in the sparking voltage depends on both the partial-water-vapour pressure and the total pressure of the mixture. The sparking potential of water vapour is found to be higher than that of dry air for the same gas pressure.