Assmann Hygrometer (or Psychrometer) in fitted wooden case with carrying handle. Made by Casella of London. Cassella Instruction Leaflet 3082/AC (which may not be relevant to this hygrometer). Cassella Instruction Leaflet 3082/AC*.
The instrument has a chromed outer casing in which is located a clockwork fan and two thermometers. Thermometer bulbs extend into metal ventilation tubes at the base of the instrument. The clockwork fan draws air through the tubes, and up to a connected shaft running between the thermometers.
It is used to measure humidity, and works on the principle of cooling due to evaporation. The bulb of one thermometer is encased by a wick which extends to the end of the corresponding ventilation tube. The wick is moistened and the fan causes an air current of 2-3 m/s past both bulbs. The difference in wet and dry bulb temperatures is utilised in calculations of the vapour pressure of water in the surrounding air.
The maker’s number on the top of the instrument (see image 3) includes a reference to BS 5248 (The British Standard: Specification for aspirated hygrometer).
A label in the case indicates that the clockwork mechanism was inspected on 03 November 1977 (see image 6).
* A Cassella Instruction Leaflet 3082/AC (which may not be relevant to this hygrometer) was not donated with the object but was found in the box of another but unrelated object.