Renault's Hygrometer

Regnault's hygrometer consists of a brightly polished thimble of very thin silver, forming the continuation of a short glass tube to which the silver thimble is attached by plaster of paris or some other cement not acted upon by ether. Through a cork fitting tightly into the top of the glass tube pass two narrow tubes of glass, one (A) going to the bottom of the thimble, the other (B) opening at the top of the vessel just below the cork; also a sensitive thermometer so placed that when the cork is in position, the bulb (which should be a small one) is close to the bottom of the thimble.

If, then, ether be poured into the thimble until it more than covers the thermometer bulb, air can be made to bubble through the liquid either by blowing into the tube (A) or sucking air through (B) by means of an aspirating pump of any sort. The passage of the air through the ether causes it to evaporate and the temperature of the liquid to fall in consequence, while the bubbling ensures the mixing of the different layers of liquid, and therefore very approximately, at any rate, a uniform temperature of silver, ether, and thermometer. The passage of air is continued until a deposit of dew is seen on the silver, which shows that the temperature of the silver is below the dew-point. The thermometer is then read, and the temperature of the apparatus allowed to rise until the deposit of moisture has completely disappeared, when the thermometer is again read. The temperature is now above that of the dew-point, and the mean of the two readings so obtained may be taken as the temperature of the dew-point, provided that there is no more difference than two or three tenths of a degree centigrade between them.

In case the difference between the temperatures of appearance and disappearance is a large one, the method of proceeding suggested by Regnault should be adopted. The first observation will probably have given the temperature of dew appearance within a degree; say the observation was 5°; pass air again through the ether and watch the thermometer, and stop when a temperature of 6° is shown. Then aspirate slowly, watching the thermometer all the time. Stop as each fifth of a degree is passed to ascertain if there be a deposit of dew. As soon as such a deposit is formed, stop aspirating, and the deposit will probably disappear before the temperature has risen 0.2°, and we thus obtain the dew-point correct to 0.1°.

The thermometer should be read by means of a telescope some 6 feet away from the instrument, and every care should be taken to prevent the presence of the observer producing a direct effect upon the apparatus.

It is sometimes very difficult, and never very easy, to be certain whether or not there is a deposit of dew on the silver, the difficulty varying with different states of the light It is generally best to have a uniform light-grey background of paper or cloth, but no very definite rule can be given, practice being the only satisfactory guide in the matter.

A modification of Regnault's apparatus by M. Alluard, in which the silver thimble is replaced by a rectangular brass box, one face of which is surrounded by a brass plate, is a more convenient instrument; the contrast between the two polished surfaces, one of which may be covered with the dew while the other does not vary, enables the appearance of the deposit to be judged with greater facility. The method of using the instrument is the same as for Regnault's,

The dew-point being ascertained as described, the tension of aqueous vapour corresponding to the temperature of the dew-point is given in the table of tensions based on Regnault's experiments,⁽¹⁾ since at the dew-point the air is saturated with vapour. We have already seen (p. 233) that we may take the saturation tension of vapour at the dew-point as representing the actual tension of aqueous vapour at the time of the experiment.

Experiment. - Determine the dew-point by Regnault's Hygrometer, and deduce the tension of aqueous vapour.

Enter results thus:

Appearance of dew: 47.1°F.
Disappearance: 47.75°
Dew-point: 47.42°
Tension of aqueous vapour: 8.28 mm

(1)	Lupton's Tables, No. 34.