F 18W/24*/BL368 (Sylvania)

18w and 36w tubes does not have identical filaments. The 18 watt filament is smaller in nature and has much lower resistance because the preheat current flowing trough it is only half of the 36 watt filament. That is the reason why two 18 watt tubes with four filaments in series on a 36 watt choke preheats well and emits electrons. what would happen if you tried to hook up two 36 watt tubes in series on one single 36 watt choke? the filaments would barely glow, if at all. The 600mm tube with high power 36 watt filaments has its own dedicated use. In this case since the voltage of the arc is the same as of 18 watt tube at 60volts, but the arc current is doubled, so the tube can be classified as high output. This was done to increase light output, or in this case, tripple the output of the UVB as these tubes were primarily used in bug zappers. With such a tube the biug zapper can be modertely sized, but at the same time be much more efficient. Also, as I mentioned before, the 40 watt tubes in 600mm flask were produced at mass in Hungary, GB and Germany, for the purpose of being compact and have double the lumen output. In the original times when they conceived this design, it was only because the factories did not want to assemble a new line to make longer tubes so they utilized the same 600mm flask for both wattages, but that also became a standard as the same 600mmm tube was much much brighter than a 20 watt tube. Since the arc voltage is the same as 20 watt bulb, the current higher, so two of the 2ft 40 watt tubes can be run on one 80 watt ballast, as was done in many street lights in GB and other countries. They idid not have huge SKZL type fixtures in GB or fi some were manufactured, they were in minute numbers, they used small fixtures with 2ft tubes inside, but those tubes were high output. Of course "high output" does mean the same thing as in US where they cram 215 watt power into a 8ft 2,4 meter tube. Those had special ceramix cathode mounts that went high up into the tube, where the filament sat almost 10 cm from the endcaps inside the tube so the mount would not melt from the beefy filament.