Barometers Realm

This Electro-mechanical precision aneroid barometer, manufactured around 1960 by Mechanism Ltd, employs an electrical method for determining the position of the free end of the aneroid capsule inside a conventional analogue barometric mechanism. The instrument was developed for the Ministry of Aviation and is covered by British Patent 816,073 (1957), which describes an innovative aneroid barometer design featuring electronic detection of the equilibrium point and a digital-style pressure readout.

The barometer is housed in a rugged, utilitarian cast-metal case with a grey-speckled finish typical of mid-20th-century scientific instruments, chosen for increased resistance to wear and corrosion. The case has an asymmetrical two-block construction: a massive rectangular base with reinforced corners connected to a cylindrical micrometric drum projecting from the side. The main block contains two isolated compartments: the upper one for the barometric mechanism, and the lower one for the electrical components and batteries.

On the front panel of the main section sits a power toggle switch with a polished metal bezel. Next to it is a small viewing window with a protective mesh, through which the cathode-ray indicator can be seen.

The cylindrical micrometric drum, machined from the same metal but finished to a higher lathe precision, carries a large knurled micrometric screw on its end face. This screw forms the regulating element that enables ultra-fine resolution of atmospheric pressure. Along the side of the drum is a window containing the cylindrical roller counter — a miniature mechanical odometer displaying pressure in millimetres of mercury. Inside the window are black-and-white numeric drums, with a red control sector to track the tens.

At the rear of the main body is a metal mounting plate with fixture holes, allowing the instrument to be secured within its original protective case. Some parts of the housing still show traces of factory sealing — red lacquer markings along the lid line.

The barometer is supplied with its original protective box made of mahogany, featuring a warm grain pattern and dovetail joints at the corners, underscoring the high quality of manufacture. A nameplate is affixed to the lid, and inside, the factory operating instructions are mounted to the inner wall. A signature chromed latch secures the lid during transport. Inside the box are guide posts and screw clamps that hold the instrument firmly in place, preventing any shifting.

The Mechanism Ltd barometer is an advanced refinement of the aneroid principle, capable of measuring atmospheric pressure with exceptionally high accuracy. It contains a stack of three sensitive beryllium-bronze aneroid capsules arranged in series. The capsules are partially evacuated and filled with a small amount of inert gas to provide temperature compensation by stabilising the elasticity of the metal. One end of the capsule stack is rigidly fixed, while the free end moves in response to changes in pressure.

A pivoted bar with jeweled bearings and a light spiral spring rests gently against the free end of the capsule stack. At the opposite end of this bar is a gold contact. It meets an opposing contact mounted on the precision micrometric screw. The screw is coupled to the external knurled knob and simultaneously drives the mechanical counter (the odometer-style scale). The counter typically carries five numeric drums, displaying pressure with a resolution of one-tenth of a millimetre. With the screw backed out, the contacts remain open and the circuit is inactive. The user slowly advances the screw until the contacts just touch. When they close, a small current flows from the battery, indicating that equilibrium has been achieved.

A miniature cathode-ray indicator — the Mullard DM70 “magic eye” — serves as the visual signal. It emits a green glow when the circuit closes, providing a clear electronic confirmation that the micrometric force and the capsule force have balanced. At that moment, display shows the exact pressure.

The high-tension (HT) circuit is powered by two 30-volt Ever Ready B123 batteries, classic post-war layered cells used in hearing aids, radio indicators, and low-power RF stages. The low-tension (LT) heater supply for the indicator tube uses a separate 1.5-volt U11-type battery.

Because the electrical system eliminates mechanical loading during measurement — the contacts merely touch, rather than transmitting force through gears or linkages — friction and backlash are effectively removed from the process. The capsule experiences practically no mechanical burden, allowing the instrument to achieve extraordinary precision.

This Mechanism Ltd electro-mechanical barometer represents a pivotal transitional technology: a bridge between the traditional analogue aneroid mechanisms of the early 20th century and the emerging era of electronically assisted measurement. By combining precision micromechanics with early miniature electronic components such as the DM70 indicator tube, the instrument illustrates the shift toward hybrid systems that would soon evolve into fully electronic sensors. In this sense, it stands as a marker of a changing epoch — one of the final, elegant steps before the dominance of solid-state barometric technology.