A rare silver, bourdon tube pocket thermometer by Francis Barker & Sons, Improved Patent No 7318, retailed by Negretti & Zambra, London, c1908

Pocket thermometer having 1¾” silvered and engraved dial, the register calibrated in both degrees Celsius and Fahrenheit with ranges 0 – 50 and 30 – 120 respectively, divided to 1/10ths. Blued steel pointer, all set beneath a bevelled glass.

Silver semi-barrel form case with engine turned finishers and verso, full English hallmarks for sterling silver, the maker Francis Barker & Son Ltd, and date letter “n” for 1908 to circumference. Vacant circular cartouche. Extension post and suspension ring.

Condition: The subject of a full mechanical overhaul. It is unfortunate that there is a scratch to the dial at roughly 11 o’clock, accompanied by some very minor marks to the centre, otherwise the dial has good colour and vibrancy the engraving clear and precise. The scratch appears dark in the images, but is, in reality, silvery, and only noticeable at certain angles. The silver case with minor marks and very light abrasions, free from dents or damage. The movement working well, showing very good sensitivity. Accuracy cannot be determined.

Comments: One of only two of these I have seen, it is not clear at all why such instruments are so rare. It might be that they were not considered accurate enough (an incorrect assessment) and that, coupled with the very substantial difference in price between a standard thermometer and this, may have ultimately led to very low production. It is noteworthy that the dial is clearly hand engraved rather than die struck, this might also point to very low production. The Bourdon tube, invented by Eugène Bourdon in 1849, became the standard as a sensor, not just in pressure gauges but also barometers, a well-recognised invention widely appreciated for its excellence. The mechanism is disarmingly simple and perhaps that is a contributory factor in its excellent function. The instrument generally crisp and in good working order.