The paper describes the construction, calibration and performance of an electrolytic hygrometer based on the principle of the “Gregory” hygrometer, but considerably smaller. Its dimensions are about 6×6×10 mm.

The element consists of two platinum-clad nickel-iron electrodes, holding a spiral of continuous fibre-glass wool yarn which is impregnated with a solution of calcium chloride. The water content of this hygroscopic salt varies with humidity, so that the resistance between the two electrodes also varies. In order to measure this resistance, alternating current of a known voltage is used, and the current is measured in terms of micro-amperes. A suitable circuit is described. Air temperature is measured by a fine thermocouple permanently attached to each element.

Long exposure to air above acid and water mixtures damages the elements so that calibration is best carried out without acid. An apparatus in which this can be done is described. A stream of air with a high water vapour content is cooled to a precisely known temperature which is below its saturation point, and then raised to the temperature required for calibration, so that its relative humidity is known as it passes over the element being calibrated. Finally the dew-point is again found as a check. Temperature affects the resistance of the elements, and calibration must therefore be carried out at three different constant temperatures.

Curves obtained by means of this apparatus show that one element will give readings from 50 to 98 per cent. R.H. within ±2 per cent. Another element with more hygroscopic salt must be used for humidities between 60 and 10 per cent. R.H.

There is an ageing process whereby the resistance of each element increases for some time after it has been constructed. This period can be shortened by exposing the elements to alternating high and low humidities for a few days. Elements which have received this treatment can be calibrated and used after six weeks, and will remain in calibration for at least a fortnight.

Subsequent calibrations can be made in a simple apparatus (which is described) involving acid-and-water mixtures, but the use of filters and exposure of the elements for no more than 15 minutes avoids damage.

The hygrometer equilibrates in a new humidity within ten minutes.

The instruments possess certain advantages: they are small, they can be read or recorded at a distance, and they do not depend upon evaporation of water, so that disturbance of the natural environment is minimised. The main disadvantages are that they cannot be exposed to saturated air without going out of calibration and that each instrument must be calibrated individually—they cannot easily be standardised for mass production.