A mechanically‐improved spark gap, in an open groove along a glass or quartz rod, is described in this paper. The output is about 0.5 cps with a flash duration of less than a microsecond. The effective source size is limited by a 1.6‐mm diam. hole in an aperture plate. Ease of assembly and replacement of faulty components is the important improvement over earlier spark gaps. Mach apparently was the first to study bullets and shock waves in air with illumination from an open spark gap. A bibliography of the history of spark discharges can be found in books by Edgerton and Frungel.