Weak field switch - how does it work?

my understanding of the weak field switch not being used in tunnels was the fact the motors can arc more thus having a greater fire risk so to speak

but who knows i could be wrong???

I don't think that LT ever expected train motors to arc and start a fire. Anyway, it is quite difficult to set fire to a cast-iron tunnel, whereas tracks in the open are surrounded with flammable trees, shrubs, fences, etc.

The explanation in books such as 1938 Tube Stock is that speed limits were lower in tunnels.

On a flat level track and all motors working it was a little over 50 but this was difficult to substantiate since the speedos were very unreliable and inaccurate. I base my estimate from driving 59s with speedos and comparing 38s - but only those which had full parallel on all motors since most on the Picc did not. We had to wait until we got the odd ex-Northern Line train.

I agree with Tubeprune. I spent a miserable year driving on the Northern when there were 38s, 59s and both variants of 72s - the stock training was interesting to say the least. I can't recall from where I get the idea but I do understand that use of weak-field flags was prohibited (everywhere on the Northern) due to full speed signal overlaps being calculated on the full speed without the weak field flag being used. Not that it stopped us trying them out so, again, I reckon Tubeprune has it about right regarding speed.

59s not in "full" parallel? I'm also intrigued.

That'll explain it - you really are a mine of ..........

The sequence light used to work, sometimes anyway, on the Northern 38s but it needed someone to put a pilot light bulb in. A lot of drivers removed the bulbs to avoid being blinded by that eerie blue glow.