VII
ON A NEW HYGROMETER
(Verhandlungen der physikalischen Gesellschaft zu Berlin, 20th January 1882.)
IN this hygrometer, and others constructed on the same
principle, the humidity is measured by the weight of water
absorbed from the air by a hygroscopic inorganic substance,
such as a solution of calcium chloride. Such a solution will
absorb water from the air, or will give up water to the air,
until such a concentration is attained that the pressure of the
saturated water-vapour above it at the temperature of the air
is equal to the pressure of the (unsaturated) water-vapour
actually present in the air. If the temperature and humidity
change so slowly as to allow the state of equilibrium to be
attained, the absolute humidity can be deduced from the
temperature and the weight of the solution. But it appears
that for most salts, and at any rate for calcium chloride (and
sulphuric acid), the pressure of the saturated vapour above
the salt solution at the temperatures under consideration is
approximately a constant fraction of the pressure of saturated
water-vapour. Hence the relative humidity can be deduced
directly from the weight with sufficient accuracy for many
purposes. And if great accuracy is required, the effect of
temperature can be introduced as a correcting factor, which
need only be approximately known.
The
The idea suggested can be realised in two ways.
instrument may either be adapted for rapidly following changes
of humidity, when great accuracy is not required, as in balance-
rooms; or it may be adapted for accurate measurements, if we