Thermometer Designs

In 1790, Professor Daniel Rutherford (1749-1819) invented maximum and minimum thermometers as a way of recording the highest and lowest temperature recorded over a certain timespan. Both instruments were simple bulb thermometers set, one above another, on a single frame.

An ivory index inside the thermometer tube marked the highest or lowest level of the liquids. In the case of the minimum thermometer, spirit was used, and for the maximum gauge mercury was employed. Rutherford's design was soon improved, as mercury tended to leak beyond the maximum index. A blued steel index in the shape of a dumbbell replaced the original ivory marker.

Bimetallic or deformation thermometers depended on different expansion rates of two metal strips set into a curl. As the temperature changed, the curvature of the metal curls also changed and moved a pointer along a marked temperature scale (Image 3).

It remains unclear when bimetallic technology began to be applied to thermometry. An early description of an English bimetallic strip of copper and iron was mentioned by Johann Heinrich Lambert (1728-1777) in his Pyrometrie oder vom Maße Feuers und der Wärme (1779). In 1767, the American astronomer and mathematician David Rittenhouse (1732-1796) described a pocket thermometer that contained a bimetallic mechanism.

The invention of the trimetallic thermometer is generally attributed to the horologist Abraham Louis Bréguet (1747-1823) who used a helical tri-metallic strip, made of platinum, silver, and gold, to produce a very sensitive thermometer. Unfortunately, the mechanism in this particular thermometer was too delicate for meteorological work, since even a small breeze upsets the instrument, and it therefore served principally as a display of Bréguet's excellent craftsmanship. The bimetallic thermometer pictured on the right was made by Bréguet and also uses a helical strip.