Originally Posted by
WHBM
It's strange to actually paint the arrow on top of the gas holders as they spiral round as they go up and down as the gas fills/empties. If you look closely at the mechanism you will see this.
The ones for London/Northolt are a design that has a static casing on the outside of the gasholder.
You're getting mixed up between 'gasometers' and 'gasholders'.
The former rotate and rise up and down, the height being used to measure how much 'reserve' gas it holds whereas the latter is just a static tank, although there must be a means of showing how much gas it is holding.
It was a 'gasometer' near (I think) St Annes radar station which used to cause confusion to radar controllers at the 'old' Preston ATCC radar unit because as it filled up and rose up, it would reflect radar echos to make it seem the aircraft were in a different position from where they actually were; when it was empty or nearly so, this reflection wouldn't happen.