Vintage 1940s F Darton Relative Humidity Meter / Precision Hair Hygrometer

This is a very nice, small (13.5cm diameter), Vintage 1940s F Darton Relative Humidity Meter / Precision Hair Hygrometer in a chrome and black cast casing with a hook for wall hanging. 

In pretty good working condition, with a few surface scuffs and scratches in the chrome and flecks of dirt / discolouration under the glass, commensurate with age and use. There is also some loss of black coating on the rear of the brass base.

I am not sure how accurate it is at measuring % Relative Humidity, and I haven't tried to calibrate it.

Approximate Size:

• Face Diameter: 13.5 cm
• Rear Diameter: 15 cm / 6"
• Depth: 4.5cm

The face and base are marked as follows:-

Range 0/ 100%

F Darton & Co. Logo on the face and rear  

Percentage Relative Humidity

British Made

Serial No. 5228

A hair hygrograph measures the humidity (or dew point). It works by using the principle that a human or horse hair changes length depending on the level of humidity in the air. This particular instrument was manufactured in the 1940s but the design dates back much earlier. They were also sold as pocket instruments, although you'd need fairly big pockets or a lab coat for this one!

The only other place where I have seen one of these instruments is online as part of the Science Museum Collection

Please click on the following link for more vintage weather / meterological instruments 
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