Precision aneroid barometer.
These were a joy to behold .......... simple read out, instant result, no interpolation or corrections, portable. The official issue was two.
The snag? Rather like owning two watches ...... if they agree, fine, if not, what to do? Thus a wise S Met O contrived to have three under his roof; if two agreed to 0.1 mb, go for it, and send the third one to the Instruments calibrators at Bracknell, pretending it was the second of two, and thus urgent.
They also obviated the lurking danger of mercury. What follows is hearsay and has lost nothing in the telling, but in essence is true. Our College at Shinfield in the lovely old RAF wartime building also taught technicians, without whom we would have become useless.
On one infamous occasion, very early in the course, and very early in the day after a heavy evening, a trainee must have missed the warning about mercury and its hazards and properties. Nevertheless our hero roused himself to catch up and began the stripping of his allocated barometer.
"All the little ball-bearings have dropped out on the floor!"