From Keith Parkin
Reading that a patent has been awarded for a melting plug that releases a warning scent in overheated circuits (3 April, p 23) reminds me that it all happened in the days of steam.
In the late 1930s the London North Eastern Railway in the UK was running steam locomotives at very near the limit at which they could lubricate the bearings. So a hole was drilled in the big end and a glass bulb of amyl acetate placed in it. The bulb would burst at 160 °F and warn of problems. How novel must a patent be?
Sheffield, South Yorkshire, UK
