Today in Technology History
March 12 (continued)

A picture of Daniell's hygrometer appears below. The image is taken from his 1823 book, Meteorological Essays.

This was not the first hygrometer; there had been many previous devices for measuring humidity. Daniell's hygrometer, however, caught on because it had a simple design and was easy to use. Here is how the 1913 edition of Webster's dictionary described it:

A form of hygrometer consisting of a bent glass tube terminating in two bulbs, the one covered with muslin [on the right below], the other of black glass and containing ether and a thermometer [on the left]. Ether being poured on the muslin, the black ball, cooled by the evaporation of the ether within, is soon covered with dew; at this moment, the inclosed thermometer gives the dew-point, and this, compared with the reading of one in the air [taken from the thermometer on the central pillar] determines the humidity.

The smaller sketch on the right labeled "Fig. 2" seems to be a simplified version of the same device.

This image comes to us courtesy of the U.S. National Oceanic and Atmospheric Administration. You can see a larger version of the image here.

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