James Six F. R. S.- Two hundred years of the Six's self-registering thermometer

Abstract

Abstract A design for a thermometer constructed to record the highest or lowest temperature during the observer’s absence was mentioned by Jean Bernoulli in a letter to Leibniz dated 1698. Not surprisingly, Bernoulli never undertook the construction of such a thermometer, for the problems involved were formidable. Various designs were proposed subsequently: in particular Lord Charles Cavendish presented the Royal Society with plans for separate maximum and minimum ‘overflow’ thermometers in 1757 (2). These were very fragile, difficult to set and use, and registered in large steps as the mercury droplets passed into the overflow chamber. However, in 1782 the Royal Society published an account of a thermometer invented in 1780 by James Six (1731-1793) (3). This remarkable thermometer recorded maximum and minimum temperatures on a single instrument, while being robust and simple to use. Its invention was considered sufficiently important for Six to be elected a Fellow of the Royal Society in 1792 (4). Since then the Six’s self-registering thermometer has played a worthy part in meteorology and oceanography and its basic design has changed little in two hundred years. Today more accurate instruments are available for professional field work but Six’s thermometer remains a popular and reliable domestic instrument, to be found in many homes and gardens. The layout of Six’s original design is shown in Figure 1. Alcohol filled the central reservoir and the upper right-hand limb of the tube, separated by a mercury sector occupying the lower part of the U-bend.