A circular polished and chrome plated mirror is enclosed on one face by a cylindrical tube with a clear window. Air is drawn into the enclosure by an small air pump either from the surrounding ambient or from another source.
The mirror is cooled electronically and its temperature is recorded by a close tolerance thermometer located at its geometric centre. The operator controls the rate of cooling by pressing a switch on the unit. The temperature at which mist is seen on the mirror is the dew point and this temperature is recorded by the operator. A second identical thermometer for recording the dry bulb temperature is also supplied. These two values allow either direct calculation of the air condition or determination of the air condition from the charts supplied.
As the mirror is electrically cooled, unlike the more traditional evaporative instruments, the process is quickly reversible allowing repeated measurements thereby maintaining a high degree of student interest.
