Exhibit of the Month February 2012
Dilatometer from Paris, circa 1810
Materials expand when the temperature increases. This property can take on significant magnitude for metals. Thus, the copper cables from overhead power lines droop in the summer, but are taut in severe frost. Users of a steel bridge hope that the engineer has taken this linear expansion into account during construction.
Tools used to determine the expansion of material samples are called dilatometers. The most common types are the thermal dilatometers. They measure a sample’s variation in length as a result of temperature increase. These devices contain a heat source, with which the temperature of the sample can be increased and a display instrument that makes the linear expansion visible.
Five oil lamps as heat source
This device is from the beginning of the 19th century. It has five oil lamps, which heat the metal sample. This takes the form of a metal rod, which is firmly clamped on one end, and the other end so approaching the sensor of the mechanical display system that it is set to zero.
When heated, the rod expands and pushes the mechanical element forward: this linear expansion is made visible on a circular scale and displayed in numeric values. With the help of the accompanying test bars of different metals, the respective expansion coefficients of the individual metal rods can be measured.
The dilatometer comes from the workshop of the brothers Louis-Joseph and Pierre Francois Dumotiez in Paris, which was known for its outstanding scientific instruments. From 1815, the workshop was managed by a nephew of the Dumotiez brothers, Antoine Hippolyte Pixii.