SM122     Differential Air Thermometer     circa 1950     Heat

  This instrument was devised by Sir John Leslie to show the temperature difference between two neighbouring locations.   The usual version is a form modification due to Matthiessen, which can be immersed in liquids.   A bent glass tube forms a manometer.   Each end is then bent again to terminate in a bulb that can be immersed in the fluid whose temperature is to be measured.   The coloured liquid in the manometer will show the temperature difference from the difference in levels in the two limbs.

  The instrument needs no calibration when it is used as a thermoscope and it will indicate the temperature difference as a movement in the coloured liquid in the manometer.

  If one of the bulbs is blackened, it will enable the thermoscope to indicate the presence of radiant heat energy.