When the pressure in the recipient to which the instrument is connected is the same as the atmosphere, the index remains at position 1 on the scale; when the pressure increases the spiral tends to open and the index moves along the scale which is calibrated from 1 to 9 atmospheres (to a quarter of an atmosphere).
This kind of instrument, which allowed the measuring of pressures up to about 35 atmospheres, was widely used in boilers, especially those of locomotives, thanks to its sturdiness and ease of transportation.
This instrument belongs to the collection of instruments purchased with a special appropriation of three thousand lire granted by the Rector of the Physics Laboratory in 1864, when the laboratory moved back to the Royal University from the Santa Teresa Lycée.