Mercury electrode switch

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 improvement wherein said envelope is formed to have two indentations located opposite one another, spaced from the ends of said envelope, and spaced at a greater distance than said terminals from said one end, said indentations projecting toward one another to define, within the region enclosed by said envelope, a gap for the passage of mercury between the ends of such region, and wherein said terminals are spaced apart in the direction of a plane which is disposed between said indentations and which is perpendicular to the direction in which said indentations project.