Why are 3.5ft tubes 38W?

[lasagafield]

4fters are 36W.
Where do the extra 2 watts go in - 1/2ft?
Are 3.5ft a legacy format or something?

[Slyspark]

They were originally 40w, then re-done as 38w

[FrontSideBus]

Higher arc voltage maybe?

[Slyspark]

I'm sure someone once told me the reason for this, but I can't remember what it was....

[SuperSix]

They seem like a bit of a strange length at first, but were designed so that with the added length of lampholders and a fitting, they'd fit perfectly into metric sized 1200mm ceiling and shelving modules.

Thorn designed them to run from common 4' 40w control gear, in hope that they'd be more successfully adopted.

Quite why they were re-rated at 38w isn't clear. The actual lamp wattage listed in the Thorn technical handbook from '78 is 39w!

Quite a few of the common old tubes are listed as actually consuming a watt or two more or less than their marked rating. Seems it was the norm at the time to round the figures to the nearest 5w.

[fluorescent]

Is it possible they were re-rated to 38w to avoid confusion with existing 40w 4' tubes?

[lampy]

they were often used in lit bus shelters on loose lamp holders on juberlee clips