The electric strength of polythene when subjected to 50-c/s alternating electric stress has been measured for a range of ambient temperatures from ????????196???????° C to 17???????° C, and that of polystyrene for temperatures between ????????196???????° C and 70???????° C. The a.c. breakdown strengths are found to be lower than the corresponding intrinsic electric strengths at room temperature and above, particularly with polythene, but no difference is observed at low temperatures. In measurements in which this lowering occurred, the results were dependent on the duration of the stress application. It is concluded that an increase in the internal temperature of the test specimens can explain these results.
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