Polarisation Phenomena in an X-ray Bulb

Abstract

HITHERTO the potential difference required to produce a discharge through a well-exhausted vacuum tube has been considered to vary only with the pressure of the gas. In the course, however, of some experiments with an X-ray bulb (where the pressure could be regulated by a Gaede pump and measured by a McLeod gauge) a continuous discharge was maintained for about eight hours on several consecutive days, and I have observed a gradual hardening in spite of the maintenance of a comparatively high pressure. Further experiments carried out in this direction have revealed a remarkable effect which takes place in an X-ray bulb or, more generally, in any vacuum tube after a sufficiently long and continuous run—an effect similar to the polarisation of an electrolytic cell. This is, that after the discharge has been kept running for a sufficiently long time through a tube (inside which the pressure is kept nearly constant) a time arrives when the current flowing through the tube begins to decrease, and finally ceases altogether. To continue the discharge it is then necessary to increase either the potential difference applied to the electrodes or the pressure inside the tube. By repeating this operation several times I could ultimately reach a stage where a potential difference of more than 50,000 volts was not sufficient to produce a discharge in the bulb, although the pressure was as high as 0.060 mm., whereas under ordinary conditions in the same bulb a much smaller potential difference was sufficient to produce a discharge under a pressure of the order of 0.001 mm. After the discharge has been stopped the bulb gradually returns to its normal condition, but afterwards a comparatively short run is sufficient to bring the bulb back to the state of polarisation.