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144 D I E L E C T R I C BREAKDOWN O F LIQUIDS E x p e r i m e n t a l Interest in the dielectric properties of cryogemc liquids continues to flourish. Two excellent articles appeared from France . Fallou and Galand (4) have studied the 50-Hz uniform field breakdown of liquid helium at 4 . 2 ° K and find that breakdown strength varies non- hnearly with electrode distance, (0 .1-3 .0 mm). At 0.1 mm, break- down strength is 45 kv /mm, and at 1. 0 mm, it is 22 k V / m m . The possible causes for this variation were discussed. Usmg divergent field geometries the authors also studied the influence of wave shape, polarity, and electrode separation on breakdown. Liquid hydrogen was the dielectric studied by Burmer and Moreau (5), who mea- sured breakdown as a function of pressure and distance between spherical electrodes. Dielectric s t r e i ^ h was found to be propor- tional to pressure in the range 1 to 5 atm. At atmospheric pres- sure, liquid hydrogen has an electric strength equal to that of a good transformer oil and is comparable to liquid helium up to an electrode distance of 0. 5 mm A 1967 paper on the electric conductivity and breakdown strength of liquid dielectrics under pulsed potential was written by Korsunovskii and Protopopov (6). Conductivity and breakdown strength of n-hexane are monotonically mcreasing functions of the content of nonpolar dielectrics, e .g . , n-octane, benzene, and m-xylene, but are irregularly alternating functions of the content of polar dielectrics, e .g . , PhCl , CHClg and C C l ^ The latter chaises were ascribed to changes in charge c a r r i e r mobility. A l l changes observed were independent of electrode material . Three articles described the erosion of electrodes due to break- down m liquid dielectrics. Goldshvartz et a l . (7) have investigated the destruction of electrodes in the electrical breakdown of liquid helium. Smgle breakdowns between a spherical W anode of 5-mm radius and a stainless steel cathode m liquid helium were studied to determme the influence of external instrumentation upon damage to the cathode surface. The other two articles were published in 1967 and were presented originally at a conference in Moscow in 1965. The f i r s t was a detailed study of the erosion of two-layer electrodes in kerosene using square pulses (8). The second studied the influence of hydrostatic pressure (up to 250 atm) on the erosion of electrode materials in transformer oil (9).
