Barometer by Jesse Ramsden

Given Jesse Ramsden’s stature as a scientific instrument maker (he was a Fellow of the Royal Society and recipient of its highest award, the Copley Medal) it is scarcely surprising that in the field of barometers he was closely involved in the search for greater accuracy of measurement. Ramsden is credited with inventing the form of pointer (or index) which enabled the meniscus of the mercury to be read without parallax, and with the ivory point method zeroing the mercury level in the cistern. He may also have been responsible for the ivory float method of zeroing, although some ascribe this to George Adams, and the temperature compensation scale designed to account for the thermal expansion of mercury.

Such features are rare in domestic (as opposed to laboratory) barometers and I was therefore pleased to discover recently a Ramsden instrument which featured no less than three: the ivory float; the double index and the compensation scale. All this is housed in a supremely elegant case with a cylindrical cistern cover.

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