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Object Overview
Bimetallic thermograph from 1920, created by Negretti & Zambra. This instrument, designed for precise measurement and recording of temperature changes, is housed in a minimalist yet elegant black steel case. Hinged glass-panelled cover of japanned metal. Despite its austere appearance, it maintains a refinement and draws attention with its construction. The steel casing not only protects the internal mechanism but also serves as a backdrop to the dynamic workings of the device, radiating a steampunk aesthetic and harking back to the industrial era.
A small glass window, embedded in the casing, allows one to observe the internal processes of the thermograph, offering a privileged view of the mechanical spectacle where each element plays its role. Through this window, you can see a large drum equipped with a clockwork mechanism with an eight-day cycle. This drum is covered with millimeter paper for graphing, neatly secured by a metal clip. The precise, rhythmic movements of the drum remind us that each moment in time is captured on paper, turning the flow of temperature into a series of graphs that tell the story of temperature fluctuations.
The slender, delicate lever connected to the writing pen also deserves attention. It seems to stretch into space, gently carrying a drop of ink on its tip. However, this is no ordinary pen — its construction is unique, differing from standard versions where ink is held in broad wings. Here, that function is performed by a tiny hollow cylinder, open at both ends. This unusual technical solution highlights the company's commitment to creating not only reliable but innovative instruments, where every movement of the pen, tracing a fine line on the paper, becomes an act of precision and elegance.
24937
bi-metallic
brass, glass, iron
The metal temperature receiver, located outside the casing, is a critical and thoughtful design choice to minimize potential data distortion. This receiver consists of a bimetallic strip, produced through hot rolling, where two layers of metal — invar and non-magnetic steel — create a unique combination for sensitive and accurate responses to temperature changes. Invar, which barely expands with temperature changes, stabilizes the strip, while the non-magnetic steel with a high expansion coefficient provides movement. As the temperature changes, the strip bends: it bends toward the invar when the temperature rises and toward the steel when it falls. This graceful deformation is transmitted to the recording mechanism, similar to how barographs work, creating a graphic record of temperature with exceptional accuracy.
The bi-metallic type has a small thermal lag and the helix operates direct on the pen arm spindle without any levers or links. The control is sufficient to overcome the friction of the pen; further the scale value is constant for even increments of temperature. The bi-metallic element is mounted on a brass bracket and iron base, and protected by a slotted brass guard.