Psy-chrom′e-ter.
A form of hygrometer invented by August.
It is on the principle suggested by
Hutton, and employed by
Leslie in his differential thermometer.
It has two very sensitive thermometers freely suspended to the same frame.
Of the two bulbs, one is wrapped with linen rags and the other is free.
The wrapped bulb is moistened and the evaporation is rapid in proportion to the hygrometric condition of the air. The evaporation reduces the temperature of the inclosed bulb and the mercury sinks.
From the different indications of
[
1814]
the two thermometers the amount of moisture in the air is derived.
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Psychrometers. |
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Psychrometers. |
a is the kind known on the Continent of
Europe as
August's psychrometer, and in
England as
Mason's hygrometer. It has a cotton-wick leading to water in a glass below the wet bulb The air circulates freely around the wet bulb.
This is now the accepted form of hygrometer, being more convenient than the
Daniell's dew-point instrument.
(See hygrometer) This is shown at
b. It consists of a bent tube with a globe at each end, and is partly filled with ether the remainder of the space with vapor of ether, the air having been expelled.
One of the tubes contains a thermometer whose bulb is in the liquid.
The stem has another thermometer as a means of comparison.
The upper globe being moistened externally with ether, evaporation takes place, producing conducing condensation of ether in the upper bulb which causes evaporation from the other in the lower bulb, and consequent fall of temperature in the liquid.
By watching the lower globe, the exact point at which dew commences to fall may be noticed, and the temperature of the inclosed thermometer is compared with that on the stem.
The precise temperature at which this moisture begins to form at the level of the surface of the ether is the
dew-point, and the difference between the
dew-point temperature and the natural temperature is the degree of dryness —
thermometric
By the use of tables prepared for the purpose, the degree
hygrometric also may be ascertained, together with other conditions of the aqueous vapor; such as its elasticity, density, weight in a cubic foot of air, rate of evaporation.
c is
Regnault's hygrometer; has a pair of glass-tubes closed at the ends by thin silver cups, containing ether; in these are thermometers whose stems rise through the corks which close the upper ends of the tubes.
Air is drawn, by means of an aspirator
d, through the tube
e, and bubbles up through the ether in cup
f, causing it to evaporate and lose sensible heat, the vapor passes with the air through the stem of the standard, and by the elastic tube to the aspirator.
The moment of the deposition of dew is observed by means of a telescope so as to prevent vitiation of the result by the warmth of the body.
A (Fig 3988) shows a form of instrument in which two indexes, that of the wet bulb and that of the dry one, are so combined that when one is placed at the hight of the mercury in a dry-bulb thermometer, and another at the hight of the mercury in a wet-bulb thermometer, a third point will indicate on a scale the proportion of moisture in the atmosphere.
The finger, whose position is a result of the two conditions stated, points on the diagram of lines, which is so constructed as to indicate the relative humidity by the upper are of degrees, the dew-point by a diagonal range of figures, and the absolute moisture by a vertical column of figures.
B is a hygrometer in which a semicircular strip of wood, having the grain running transversely has its convex side coated with water-tight cement.
The free end of the strip is connected to a registering apparatus which indicates the humidity by the expansion of the exposed side of the strip.
