are cut to overlap and be sandwiched between the associated ends of the partition assembly 14 and the support disc 24 and end wall 13 of the envelope respectively short circuiting of the arc along the longitudinal edges of the panel segments 15 is avoided by forming such edges into laterally extending tabs 28 that are seated against the envelope walls this increases the 34 arc gap 34 distance along these junctures and thus provides a barrier to arc penetration before the envelope 12 is hermetically sealed it is evacuated charged with a suitable fill gas such as argon or a mixture of argon and neon at a pressure of several torr and then dosed with mercury through a tubulation not shown in the usual manner to provide an ionizable medium that sustains an electric discharge when the lamp 10 is energized as shown in fig3 the panel segments 15 except the one which is adjacent to and extends between the electrodes 16 and 17 are provided with apertures 30 at alternate ends so that the envelope sectors are interconnected and define a continuous discharge channel that traverses the envelope 12 in retroverted fashion from one of the electrodes to the other since the ends of the partition assembly 14 are firmly seated against the compliant pads 26 and 27 which in turn are seated against the support disc 24 and envelope end wall 13 the core chamber c is closed off from the electric discharge and thus displaces or 34 pushes 34 the arc away from the central portion of the lamp 10 toward the arcuate walls of the envelope 12 the.