This simple, inexpensive instrument measures the rate of evaporation from a paper surface kept constantly wet. The results obtained cannot be related quantitatively to natural evaporation from ground or plant surfaces, or from free standing water, but they provide a convenient method of comparing evaporation rates at different places in the same locality, or in one place at different times.
A graduated glass tube filled with distilled water is inverted with a filter-paper disc clipped over the open end. The rate of water loss is noted. The instrument may be hung from a stand or fitted in a meteorological screen for wind and dust protection.