Details are given of the design and method of fabrication of double-walled permanently evacuated metal siphons suitable for carrying liquid air, liquid hydrogen, etc. When properly constructed these siphons require no attention and can be used as standard elements in low-temperature plant, using ordinary `petrol unions' for making connexions between siphons. The loss of liquid due to heat influx across the vacuum space is very low. Even with a clearance of only a few millimetres between the two tubes making up a siphon the outer wall remains only a few degrees below room temperature during use.
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Jones et al. (1948) studied this question.