You are discribing a hygrometer or wet and dry bulb thermometer.
You also need a chart which tells you the relative humidity corresponding
to the difference between the wet and dry bulb reading. This instrument
is not as accurate as a sling psychrometer which also uses a wet and
dry bulb, but is rigged so that the thermometers are rotated very
rapidly around an axis by means of a handle (originally they were "slung"
around on the end of a cord or rope).
Bob Schramm
----------