Abstract: In a mercury switch composed of a closed tubular glass envelope and at least one pair of electrodes extending through one end of the envelope and presenting terminals located within the region enclosed by the envelope and adjacent the one end thereof, the envelope is formed to have two indentations located opposite one another, spaced from the ends of the envelope, and spaced at a greater distance than the terminals from the one end, the indentations projecting toward one another to define, within the region enclosed by the envelope, a gap for the passage of mercury between the ends of such region.