The breakdown characteristics of capillary tubes are investigated experimentally and theoretically. Using a 6301 type standard calibrating system

the breakdown characteristics of capillary tubes are measured in six gases （H2. N2. He. Ne. Ar

Kr） within the pressure range from 10-1 to 20 torr and they are compared with those of the parallel electrode systems and of the small-area electrode systems free from wall effects. The results show: （1） the breakdown voltage of a capillary tube is always higher than that of the ideal parallel electrode system

provided the spaces between electrodes are equal; and （2） Paschen’s Law is no longer valid for the case of capillary discharge. A theory considering wall effect is presented

and the breakdown conditions are derived. Theoretical and practical results are to some degree in agreement.