​In 1949, Paul Hiltz received a US patent for a hygrometer design that used the hygroscopic properties of a goldbeater's skin membrane to measure relative humidity. Goldbeater's skin is made from cow intestine.  The patent was assigned to Serdex, Inc of Boston, Massachusetts.  Bacharach made hygrometer under the Serdex patent.  I'm not sure of the date, the presence of a US "ZIP code" postal code on the meter face dates it to 1963 or later.  The meter face is about 3.75 inches wide. It still works well.

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