SM119     Daniell’s hygrometer     circa 1960     Heat

  This is an apparatus relying on the dewpoint to measure humidity in the air.   It consists of two glass bulbs connected by a glass tube bent at right angles twice.   One of the bulbs, which is at a lower level, is two thirds full of ether and has a sensitive thermometer immersed in it.   The apparatus contains only ether or its vapour.   The other bulb, covered with muslin, is cooled by dropping ether on the outside and letting it evaporate.   This causes ether from inside the lower bulb to distil inside the cooled bulb causing the temperature of the lower bulb to fall.   When the temperature falls to the dewpoint, water from the atmosphere condenses on the outside.   The dew is made more visible by means of a silvered ring on the bulb.   The temperature at which dew first forms is noted and then the bulb is allowed to warm up.   The temperature of the bulb when the dew disappears is again noted and the average is the dewpoint.   The dewpoint can be converted into percent relative humidity at the ambient temperature by using dewpoint / vapour pressure tables.

  Sources of error in this apparatus are:

  (a)     Only the surface of the ether is cooled so the thermometer might not give the exact temperature.

  (b)    The ether vapour from the cooling process might affect the ambient conditions.

  (c)     Bad thermal conductivity of glass can cause an error in the dewpoint reading.