Barometers Realm

The thermograph is tastefully housed in a mahogany case, glazed on three sides. Its hinged top lid features a convenient carrying handle and two latches on the front to secure it to the base. All the fittings are made of nickel-plated brass. On top of the lid, there is a celluloid plate engraved with information about the instrument and its maker.

Inside the case, a brass drum is driven by an eight-day clockwork mechanism. The temperature chart is recorded on special graph paper, held in place on the drum by a brass clip. The recording pen is mounted on a delicate lever, which is connected to the sensitive element.

The sensitive element is located outside the main case, protected by a metal mesh cover that resembles an open-ended cylinder. It consists of a short, curved Bourdon tube — about one-third the length of a standard one — made from high-grade alloy steel. The tube is filled with a liquid that has a high coefficient of thermal expansion. As the temperature rises, the tube lengthens and expands, its volume decreases, and the curve becomes more pronounced. The change in length is minimal due to the short form of the tube, so it doesn't interfere with the measurement.

A long rod made of the same material extends from the Bourdon tube and connects via a linkage to the lever inside the instrument — the one carrying the recording pen. This linkage transmits and amplifies the motion caused by the bending of the tube in response to temperature changes. The Bourdon tube and rod are mounted on a brass frame, which includes a screw wrapped in a helical spring at the top for calibrating the scale. A small latched access window in the protective cover allows easy access to this adjustment screw.

The invention of the Bourdon tube by engineer Eugène Bourdon was a true breakthrough in the world of precision measurement. For the first time, it became possible to create an entirely metallic manometer — with no mercury involved! It turned out to be the perfect solution for monitoring steam pressure in industrial boilers: more compact, more reliable, and, most importantly, safer. And all of this came just as steam power was rapidly becoming the driving force of the industrial age.

Bourdon filed a patent for his invention in Paris on June 18, 1849. Interestingly, the patent described not only the pressure gauge itself but also the potential to apply the same principle to barometers and thermometers. The core idea was brilliantly simple: a curved, slightly flattened metal tube bends when the internal pressure or temperature changes, and this movement can be transferred to a pointer or recording mechanism.

Although originally designed for industrial use, the Bourdon tube quickly found its way into scientific and meteorological instruments — including thermographs like this one. Its mechanical simplicity and high sensitivity made it a true gem for accurate measurement.