25-04-2017, 03:41 PM
(25-04-2017, 09:50 AM)Terry Wrote: Most of the Underground trains I used when I was at school in the late 50s had fluorescent lighting with one incandescent light at each end of the coach. Every time the train went over a section gap, all the fluorescents went out, leaving just the battery backed incandescents alight. The fluorescents would all then strike again after the car cleared the gap........
I was always fascinated by those fluorescents operating on DC. ISTR the tubes were connected in series across the 600V supply with a single resistive ballast. Presumably the ballast had to be high enough resistance to stabilise the series circuit with reasonably constant current. With the tube discharge having its usual negattive resistance, the design solution for ensuring economical balanced operation of series-connected tubes has never been obvious to me, unless of course they used a few KT66s providing constant current .
The classic discharge striations as the line voltage fell could be very entertaining. There was also the uneven blackening of the tube, I guess due to evaporated tungsten ions migrating from the filaments to the negative terminal.
Such a lot of physics to ponder on one underground journey! Are there any DC-operated fluorescents left for the delight of the next generation of physiscists?
Martin
BVWS Member
