Dines's Hygrometer

Dines's Hygrometer is an instrument for directly determining the dew-point, i.e. the temperature at which the air in the neighbourhood of the instrument is completely saturated with aqueous vapour. It consists of a thermometer placed horizontally, so that its stem is visible while its bulb is enclosed in a box of thin copper through which cold water can be passed from a reservoir attached to the instrument by turning the tap at the back. The tap is full on when the side marked o is upward, and shut off when that marked s is upward. The bulb of the thermometer is placed close to the top of the box which encloses it, and the top of the box is formed of a plate of blackened glass, ground very thin indeed, in order, as far as possible, to avoid any difference of temperature between the upper and under surfaces, and so to ensure that the temperature of the thermometer shall be the same as that of the upper surface of the glass.

The temperature of the box is cooled very gradually by allowing water, previously cooled by adding ice, to pass very slowly from the reservoir along the tube. As soon as the surface of the glass is at a temperature below that of the dew point, a deposit of dew can be observed on it. This can be easily noticed by placing the instrument so that the glass surface reflects the light of the sky, and accordingly presents a uniform appearance which is at once disturbed by a deposit of dew. The temperature t, say, at which this occurs is of course below the dew-point. The film of moisture is then allowed to evaporate, and when all has disappeared the temperature is again read - let it be t'. This must be accordingly above the dew-point. Now allow the water to flow only drop by drop, cooling the surface very slowly indeed, and observe the same phenomena again, until t and t' are not more than one or two tenths of a degree apart. Then we know that the dew-point lies between them, and by taking the mean of the two obtain an accuracy sufficient for practical purposes. The fall of temperature can in some cases be made so slow that a fugitive deposit forms and disappears at the same temperature, in which case the temperature of the dew-point is indicated by the thermometer as accurately as the variation of the quantity to be observed permits.

It is important that the observer should be as far as possible from the glass surface during the observation, in order to avoid a premature deposit of moisture. To this end a telescope must be mounted so as to read the thermometer at a distance, placing a mirror to reflect the scale of the thermometer to the telescope.

We may thus determine the dew-point, but the usual object of a hygrometric observation is to determine the tension of aqueous vapour in the air at the time of observing. We may suppose the air in the neighbourhood of the depositing surface to be reduced to such a state that it will deposit moisture, by altering its temperature merely, without altering its pressure, and accordingly without altering the tension of aqueous vapour contained in it. We have therefore, only to look out in a table the saturation tension of aqueous vapour at the temperature of the dew-point and we obtain at once the quantity desired, viz. the tension of vapour in the air before it was cooled.

We may compare the result thus obtained with that given by the wet and dry bulb thermometers. In this case the observation consists simply in reading the temperature of the air t, and the temperature t' of a thermometer whose bulb is covered with muslin, which is kept constantly moist by means of a wick leading from a supply of water. The wick and muslin must have been previously boiled in a dilute solution of an alkali and well washed before being mounted, as otherwise they rapidly lose the power of keeping up a supply of moisture from the vessel

The tension e" of aqueous vapour can be deduced from the observations of t and t' by Regnault's formula⁽¹⁾ (available when t' is higher than the freezing point)

where e' is the saturation tension of aqueous vapour at the temperature t', and b is the barometric height in millimetres.

Experiments. - Determine the dew-point and the tension of aqueous vapour by Dines's Hygrometer, and also by the wet and dry bulb thermometer.

Enter the results thus:

Appearance of dew: 47.1°F.
Disappearance of dew: 47.75°
Dew-point: 47.42°
Tension of aqueous vapour deduced: 8.28 mm
Tension of aqueous vapour from wet and dry bulb: 8.9  mm

(1)	1 The reduction of observations with the wet and dry bulb thermometers is generally effected by means of tables, a set of which is issued by the Meteorological Office. The formula here quoted is Regnault's formula (Ann. de Chimie, 1845) as modified by Jelinek. See Lupton, table 35.