Research on multiply-ionized atoms was pioneered in the 1930s and 40s, particularly in Sweden by Edlén and his colleagues working first in M. Siegbahn's laboratory in Uppsala, and then at Lund University. The key instrument was the grazing-incidence grating spectrometer which opened to observation the spectral range 500–5Å (Edlén 1963). The source of the multiply-charged ions was the plasma of a vacuum spark, a pulsed (~1 µs) electric discharge of up to 50 kA between electrodes of the desired element, some 3–5 mm apart. When the spark was enclosed in a ceramic tube (with a side opening to let out the radiation), the breakdown voltage was less, the discharge was more controllable. Furthermore, it was found that the range of species produced was a useful function of current.