Breguet's thermometer

[2] The strips of soldered metals are curved into a helix (a).

The upper extremity of the helix is fastened to a metallic support (c) and the lower extremity is connected to an index, which projects over a graduated circle (b).

The expansion of silver with temperature is almost twice as great as platinum, with gold being somewhere in between.

The result is that a temperature rise or fall will cause a corresponding twist in the spiral, moving the index.

The slip of gold in between is to prevent "sudden starts".