Pyrometers are used to demonstrate, and sometimes measure, the amount by which a metal rod expands as it is heated. The device, which is also called a dilatometer, was invented by Pieter van Musschenbroek (1692-1761).

   Cadran's Pyrometer, at the right, above, had a gas burner underneath the expanding rod at the top. The right end of the rod is fixed, and the left end actuates a lever mechanism which amplifies the motion and causes a needle to swing around the semicircular scale. This apparatus at Amherst College was made by E. Ducretet & Cie. of Paris, and cost 50 francs ($10) in the 1879 catalogue.

   The instrument at the right, below uses an alcohol burner to heat the expanding rod. It is in the Smithsonian collection, and was made by Golaz of Paris ca. 1875.

   All of these instruments can be used only for relative measurements, as there is no way to measure the temperature of the expanding rods.

   This pyrometer is in the Smithsonian Collection and was made by Pixii of Paris. It is listed at 140 francs in the 1849 Pixii catalogue, equipped with copper, silver, iron, brass and steel rods of the same length and diameter. 
  The pyrometer at the right was made by Max Kohl of Chemnitz in Germany, and was purchased by the College of Wooster in Ohio about 1900. Curiously enough, it was bought to replace apparatus lost in a fire.

   The gas jets used to heat water in the copper container are across the bottom of the apparatus.

   This pyrometer is clearly home-built, and resembles the Pixii apparatus above. 

   It is in the Millington/Barnard Collection at the University of Mississippi Museum. 

   There is other apparatus in the collection built by John Millington, the first Professor of Natural Philosophy at Mississippi, 1848-1853; could this piece have been built by Millington?

   The pyrometer at the right is in the Greenslade Collection, and is listed in the ca. 1900 Max Kohl apparatus catalogue, without the gas burner, at 24 marks (about $6).

The thermal expansion apparatus below was popular in the first half of the 20th century, but seems to have fallen into disuse since then. This example was sold by the W.M. Welch Scientific Company of Chicago for $8.80 in 1928. The metal rod under test is held between a fixed and a moving point inside the jacket through which water of various temperatures was passed; the tube at the top originally contained a thermometer in a rubber stopper. The amount of expansion was measured with the micrometer screw on the left-hand side that was advanced until it just touched the free end of the rod; this completed an electrical circuit and an external battery and bell was used to indicate when contact was made. The nominal length of the rod is 60 cm. This apparatus is in the Greenslade Collection.

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