Aspiration psychrometer conceived by the meteorologist Richard Assmann in 1892.

Psychrometers are measuring instruments designed
to determine the relative humidity of the
atmosphere in the vicinity of the instrument.
The humidity value is calculated from the
temperature difference (psychrometric difference)
between two different thermometers.
The first thermometer measures the dry ambient
temperature, the second thermometer measures the
temperature of a moistened thermometer. Through
the evaporation of water, evaporation heat is drawn
off the wet thermometer until a balance between the
heat which has been drawn off and the heat from the
surrounding air has been attained. This is the wet
temperature which is lower than the dry. The
temperature of the wet thermometer decreases with
increasing evaporation, i.e. the air is drier. Both
thermometers are ventilated. The most accurate
results are attained when an air velocity of at least 2
m/s prevails around the thermometers.
(see http://www.fischer-barometer.de/datenblaetter/E430101.pdf)

Serial No 321363