Psychrometer

You can use a psychrometer to measure relative humidity.

A sling psychrometer is used to measure relative humidity.

1982 July, Ray W. Brown, Dale L. Bartos, A Calibration Model for Screen-caged Peltier Thermocouple Psychrometers, United States Department of Agriculture: Forest Service, Research Paper INT-293, page 2, Gross adjustment of the model using a small number of well controlled calibration measurements would suffice to make the model applicable to any new psychrometer or group of psychrometers.

2007, N. J. Livingston, G. Clarke Topp, 71: Soil Water Potential, M. R. Carter, E. G. Gregorich (editors), Soil Sampling and Methods of Analysis, 2nd Edition, Canadian Society of Soil Science, Taylor & Francis (CRC Press), page 976, Psychrometers have two main components: the porous cup with its contained sensing and reference junctions and the instrument for generating the electrical current and measuring the psychrometer output.

2008, Ross Montgomery, Robert McDowall, Fundamentals of HVAC Control Systems, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Elsevier, page 132, An aspirated psychrometer combines low cost with the fundamental measurement principle of wet- and dry-bulb readings.