The instrument makes use of two simple crystal-controlled oscillators having nearly identical temperature/frequency characteristics. One of the quartz plates is covered with a very thin hygroscopic film and exposed to the atmosphere. The difference in frequency of the two oscillators is a function of humidity. Typical results are 6.5 Hz per % change in humidity at room temperature. The hygrometer resolves changes in humity down to 4/100%, the time constant of the sensor being better than 1/50s.