The breakdown voltages of five Freon gases and SF6 were measured at low pressures between parallel plates to determine their insulating properties under these conditions, and especially to find their, minimum breakdown voltages. It was found that the advantages of these gases over air are much less at low pres¬sures than at atmospheric pressure or higher and that the use of these gases as insulators at low pressures is therefore limited. Preliminary investigations revealed, however, that the vapors of compounds that are liquid at normal temperature and pressure, such as FC-75, maintain their dielectric strength much better than do electronegative gases as the pressure is reduced, and thus seem to be promising for use as insulators at low pressures.
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