Honeywell Pressure Switch
This pressure switch came from Pirelli Tyres in Carlisle and was given to me by John Wilson. John was the site services engineer for many years. He was cleaning out his locker when he came across this old switch.
As can be seen from the above pictures the switch has a glass tube filled with mercury. When the requested pressure is reached the glass tubes tilts causing the mercury to flow over the contacts and makes the circuit.