Experimental details are presented of an excitation unit developed for use in producing the rare gas continua in helium, argon, krypton, and xenon. The unit is essentially a thyratron-controlled modulator which replaces the spark-gap external trigger in a conventional spectroscopic 'condensed discharge'. With this unit, it is possible to obtain the Hopfield helium continuum in the 580 to 1100A wave-length region with improved intensity and stability so that an improved bandwidth of slightly less than 0.25A is obtained with a 2.2 m normal-incidence vacuum monochromator and photoelectric scanning detection. This excitation unit is described, and its application to study of the rare gas continua and to absorption cross-section measurements in hydrogen is illustrated. (Author).