The essential point about the lamp described in this paper is that it consists of an open U tube. Clean dry mercury is poured into the tube and direct E.M.F. applied by iron electrodes dipping into the mercury in each limb. A portion of the U tube must be capillary. On vaporizing the mercury in the central part of the capillary tube an arc can be started and maintained in the capillary. Details of the construction and behaviour of such a lamp are given. The lamp can be constructed to operate on any direct voltage from 100 upwards, takes very little current, and has so far as we know no limit to its working life.
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Vincent et al. (1924) studied this question.