I wonder just how many of these were actually made ??
By act of parliament every mine in the UK had to have one also public buildings, prisons and hospitals were also required to have them.
Negretti & Zambra were just one of at least 6 different instrument manufacturers in the UK who produced these devices in their thousands yet, surprisingly, very few seem to have survived the passage of time.
I just could not resist purchasing this one for the princely sum of 50 pence.
Tweakie.