The current RH is shown directly by a hygrometer, whilst a hygrograph records the value on a chart. An example is a hair hygrograph, which depends on the extension of fibres resulting from their increased absorption of moisture in high humidities; Horace de Saussure (1740-99) found that human hair extends by about 2.5 per cent between extremes of dryness and wetness. But the length changes are not quite proportional to moisture variations, so the RH scale is irregular on the recording paper.

By the way, a hygrometer is quite different from a hydrometer, which measures the density of liquids and is used to check the battery fluid or radiator fluid in a car, for instance.

There are several sources of error in taking readings with a hygrograph. Dust can cause errors of up to 15 per cent, so the hairs should be washed frequently. The hairs should not be touched by hand, in case they become greasy. Even in perfect condition, a hygrograph may still have errors of 5 per cent, and yet more when cold or in a very dry region. There is also a lag on sudden changes of humidity; a good hygrograph normally adjusts to 90 per cent of any abrupt alteration within about three minutes, but it takes much longer at low temperatures. The instrument should be tapped lightly prior to taking a reading, to overcome friction of the pen on the paper.

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