Electrical Breakdown of Ice at Cryogenic Temperatures
An ice or ice-silica compound system is attractive for electrical insulation at cryogenic temperatures. A sample of ice without silica was found to have a breakdown strength of 0.3 MV/cm for ac voltages, and 0.6 MV/cm for impulse voltages. The breakdown strength of an ceicesilica compound system was 0.7 MV/cm for ac voltages. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids may exist, since partial discharges were observed at about half the value of the ac breakdown voltage. The relationship between partial discharges and microscopic voids, and the voltage-time characteristics of this insulation system were studied.
Dielectric Properties of Cryogenic Liquids
Electric-breakdown measurements at 60 Hz have been made with cryogenic liquids-nitrogen, hydrogen, and helium. All of the cryogenic liquids have higher breakdown voltages than conventional transformer oil, except liquid helium which breaks down at a considerably lower voltage. The measured values of dielectric constant for the cryogenic liquids agree quite well with the published literature. The dissipation factor of the cryogenic liquids is so low that true values cannot be reported. They appear to be below 0.000001. However, by comparison of the indicated values, it appears that the dissipation factor of liquid helium may be about ten times that of liquid hydrogen.
Détermination de la constante dielectricque du deuterium liquide
Resume Nous avons fait des mesures sur la constante dielectrique du deuterium liquide entre 18° et 22°K. Des mesures de controle ont ete faites sur l'hydrogene leger liquide. Ces derniers resultats sont en bon accord avec les valeurs obtenues par W o l f k e et K. O n n e s. A partir de nos valeurs obtenues pour la constante dielectrique du deuterium et de l'hydrogene, nous avons calcule les expressions e— 1 e + 2 1 ρ , entrant dans la formule de C l a u s i u s-M o s o t t i. Nous avons trouve pour ces expressions des valeurs constantes a 0,2% pres. Moyennant ces dernieres valeurs, nous avons calcule les polarisibilites pour D2 et H2. Nous avons trouve respectivement les valeurs suivantes: 1,985 et 1,991. Les erreurs relatives pour ces valeurs sont estimees egales a 0,05%. On constate donc vue la polarisibilite de l'hydrogene leger depasse d'environ 0,3% celle de l'hydrogene lourd. Ceci serait en accord avec des calculs faits par M. B o u c k a e r t. Les resultats de ces calculs seront publies recemment.