For sale, a Victorian Mason’s Hygrometer or Wet & Dry Thermometer on stand by Negretti & Zambra.
The instrument is comprised of two thermometers affixed by four brass clasps to a rectangular brass plate with two solid brass and silvered scale plates affixed above and below each thermometer respectively. Both have an engraved Fahrenheit scale registering 20 to 120 degrees. The maker’s name of Negretti & Zambra London is engraved to the centre of the backplate with “dry” to the top right and “wet” to the bottom left to denote each thermometer’s use.
The thermometer plate is held against the weighted stand by means of a slotted section for the base and a screw to the top which threads into the top for security. Below the plate a brass ring is provided to hold the original glass water vessel which feeds a consistent supply of water onto the wet thermometer bulb by means of a piece of cloth.
Meteorological instruments such as these have a number of associated names, hygrometer meaning the measurement of moisture or water, the name psychrometer (cold meter) was used by the German inventor Ernst Ferdinand August (1795-1870) and during the same period in 1836, the Surgeon, John Abraham Mason presented his new, Mason’s Hygrometer. An article in the Record of General Science describes a “new hygrometer illustrated by experiments and a comparison of its results with Sir John Leslie’s”. Leslie had in 1805 developed an air thermometer based upon Galileo’s thermoscope. Sadly, Mason’s name is no longer commonly associated with his invention but its continued manufacture is testament to its superior accuracy.
It is now more commonly dubbed, the wet and dry thermometer relating to the comparison of two thermometers, one with a wet bulb and the other dry. This is essentially the same as Mason’s pattern where one thermometer is kept wet by means of a water soaked sock covering one bulb and the other kept dry. The evaporation of water from the sock has the effect of lowering the temperature reading on the wet thermometer and the difference against the dry thermometer is used to calculate relative humidity of the air.
A design very similar to this hygrometer is provided in both the Negretti & Zambra catalogues of the 1870’s and 1880’s and is described as a “Portable Wet & Dry Bulb Hygrometer”. The description states:
“A most compact and convenient form of hygrometer, invented by Negretti & Zambra, as a companion instrument to their small Patent Maximum and Minimum Registering Thermometers and Pocket Aneroid Barometer”.
Negretti & Zambra were a leading name in the production of meteorological and scientific instruments and have a company history dating back to 1850. Throughout their long and esteemed history they exhibited at British industrial fairs throughout and became makers to both Queen Victoria and Edward VII. Owing to changes in the business, the firm ceased the public retailing of scientific instruments sometime around the late 1960’s and continued with a focus on the aviation industry in numerous guises until its eventual liquidation in the year 2000. A sad end to a hundred and fifty years of quality manufacturing.