A thermoscope is a device for detecting and displaying temperature changes. In demonstrations I use an Erlenmeyer flask containing a centimeter of water. A long, narrow-bore glass tube passes through a rubber stopper in the mouth of the flask and almost touches the bottom of the flask. Blowing gently into the open end of the tube brings the water level up into the upper part of the tube. Placing my hand on the side of the flask causes the water level to rise rapidly in the tube. 

   The differential thermoscope has two thin spherical bulbs connected by a U-shaped tube. The closed system is filled with dry air, and the two segments are separated by a small slug of mercury in the horizontal part of the tubing. In use, the two bulbs are placed the same distance from two sources giving off radiant energy at different rates, either because they are at different temperatures or because they have different emissivities, the movement of the mercury slug is observed. In the usual demonstration, a Leslie's Cube is placed between the two bulbs. 

   This example is at Washington and Jefferson College in Washington, Pennsylvania
 

This differential thermoscope with legs of different lengths is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. The length of the longer arm is 20 cm, and the shorter arm is 9 cm in length.

   In his 1983 Catalogue of the Collection, Prof. Robert T. Lagemann wrote: "The delicate thermoscope is placed in a cylinder filled with water, so as to cover the uppermost bulb, and then a container of hot liquid, perhaps oil at a temperature of two or three hundred degrees, is placed in the upper part of the water. Heat from the hot oil will penetrate the water and then the upper bulb, whereupon the increased vapor pressure will cause the index liquid [mercury, for example] to move. The fact that very little movement is noted after a considerable time has elapsed shows that liquids are very poor conductors of heat."
 
 
 

   It is part of the Millington/Barnard Collection at the University of Mississippi in Oxford, and is on display at the University Museum. 
 
 

   The differential thermoscope also forms part of  Ritchie's Apparatus.