4 pin potentiometers, 4th pin what for?

[JMMP]

Replaced a bunch of these (6x15mm, marshall jcm2000 for instance), 3 of the pins do what I'd expect, the 4th outer pin gives no reading one way or another, anyone know what this 4th pin is there for?

[Leo_Gnardo]

I've wondered that myself. Far as I can tell the 4th pin is a sort of "vestigial organ". Lucky for us it can't cause trouble. Possibly the manufacturer has made provision for a tapped pot should anyone order such a thing.

[g1]

I bet if you look at the datasheet for that particular series of pots, there will some optional function (like the tap Leo mentioned) for the fourth leg. In this case it is not used, but it will be in other types from that series.