Info needed: Foam extinguishers fitted on trucks

[Allan L]

Internally, at the top, is a small container of 'something' which when tipped upside down, spills into the main container which holds 'something else' - the resultant chemical reaction creates a foam which is discharged via the hose thingo.

PS That exhausts my technical knowledge of those things. So I guess that violent vehicle movements might see some accidental discharges.

Edit: It's coming back, I think. The small container near the top I think was somewhat bottle shaped (perhaps even made of glass??), with a lead stopper to prevent accidental spillage, so needed to be tipped upside down to displace the stopper. Wikipedia probably has a story on them.