w 41 lamps are spaced at 3 meters from each other in a broilers house 9 w 66 lamps are spaced at 6 meters from each other in a broilers house the greater spacing is possible due to the nearly monochromatic spectrum of this lamp nearly monochromatic green color 11 w 41 lamps are spaced at 3 meters for the breeders when applying the above examples in closed broiler houses the lights should be on for 23 hours daily during the first week and 16 18 hours daily after the first week referring to fig2 there is shown a circuit diagram of a preferred dimming device 4 including a delay unit 22 e g a timer governing the position of the switch 24 from a first position wherein the entire voltage of the mains is applied to the power supply line 14 16 activating the lamps to a second position contacting point 26 as indicated by the broken line 28 leading from the delay unit 22 to the switch 24 there is further included a selectable step down voltage unit 30 e g an electromagnetic or electronic transformer having a first tapping 32 providing a first power rating of e g 185v 8 a and a second tapping 34 of e g 155v 6 a a first manually operated switch 36 is interposed between the terminal 6 connectable to the mains and the switch 24 a second manually operated switch 38 is located to selectively connect between tappings 32 34 and contact point 40 in order to assure electrical continuity of the dimming device during the changeover of switch 24 there is provided a ballast 42 connected between the step down voltage unit 30 and the output terminal 14 across the switch 24 there are advantageously connected a.