Barometers Realm

This Luftwaffe Bakelite Height Recorder, circa 1940, manufactured by Gebr. Winter, is a height-recording instrument — essentially a barograph, that is, a device measuring variations in barometric pressure, interpreting them as changes in altitude, and recording those changes on a rotating drum driven by a clockwork movement.

The housing is a solid, straight-sided box made of dark Bakelite with a characteristic brown-amber “walnut” mottled pattern. The front features a large viewing window closed with a transparent panel through which the altitude drum is visible. A metal pull-handle is fixed to the back; pulling it fully withdraws the internal chassis like a drawer, providing access to the drum, the aneroid system, and the clockwork. This construction made it possible to replace chart paper quickly and service the instrument with ease.

Inside, the entire mechanism is mounted on a frame made of low-carbon steel, finished with hammer-tone enamel over a zinc primer. The surface has the familiar pebbled texture — a durable anti-corrosion coating typical of wartime aviation equipment of the 1940s. The frame carries the chart drum with its clock drive, the aneroid assembly, the linkage system, and the stylus holder.

The principal sensing element is a pair of self-springing aneroid capsules joined into a single stack. The material is phosphor bronze with factory passivation. The colour is unusual — a muted, graphite-grey matte tone, almost zinc-steel in appearance — the result of controlled oxidation that reduces corrosion and stabilises the elastic modulus. The deep stamping of the diaphragms indicates a ductile alloy with resilient, reversible deformation and excellent resistance to long-term creep — essential properties for precision altimeters operating under vibration and temperature fluctuation.

Engraved beneath the drum is the name ISGUS. This is a German company from Schwenningen, founded in 1888 as J. Schlenker-Grusen (later shortened to ISGUS). Known for time-recording mechanisms, industrial clocks, and work-time control systems, ISGUS supplied precision drum movements widely used in scientific and aeronautical instruments.

Also on the underside of the drum are the winding key and the duration selector. Three settings — 2, 4, or 10 hours — determine the full rotation period. A single full wind provides approximately 12 hours of continuous operation. To start the instrument, the mechanism must be switched from Aus to Ein — only then does the drum begin to rotate and register altitude. A paper chart is mounted onto the drum; as pressure falls (altitude increases), the aneroid capsule raises the stylus along the scale, producing a continuous trace.

Such height recorders were typically mounted in aircraft or airships using shock-absorbing rubber bungee cords to minimise vibration and turbulence. For this purpose, the body features dedicated mounting lugs at both the top and lower edges, allowing secure suspension. Today, the instrument stands as a rare example of Second World War aviation metrology — the convergence of Bakelite field design, utilitarian engineering, and precision mechanics — preserved as a complete historical artifact.