Ancient Thermometers.

Kaipara and Waitemata Echo, 25 September 1914, Page 8

Ancient Thermometers.

The earliest thermometers were of the-expansion type, and were due to the exertions and research of Galileo. They consisted of a glass bulb with a capillary tube attached, the bulb being filled with alcohol, which expanded with temperature, the expansion being indicated by the movement of the alcohol up or down the capillary. The scale on the thermometer was purely an arbitrary one, and it was not until Hooke, in 1681, suggested the melting-point of ice and the boiling point of water that any attempt was made to obtain a fundamental scale of temperature. Thirty years afterwards, in 171-1, Fahrenheit suggested his scale. As the Fahrenheit scale is still the one almost universally used by English and American engineers, it may be of interest to know how Fahrenheit, obtained his scale. The explanation rcenernily accepted is ! due to the late Dr. Gam gee. The lower fixed point of the scale was i determined by a mixture o;' <■ m >\v . and salt, and the upper by placing a thermometer unrlc-r tho armpit or | inside the mouth of a healthy man. In the earlier thermometers (says "Engineering") the interval bet■■•■ <-en these tw!o fixed points was divided into fourths, making 96 parts. It was subsequently found that, by a pure coincidence, the 32deg. corresponded to tho melting-point of ice, and the 212deg. to the boiling point of water. The centigrade scale was introduced by Celsius, '■ and is, according to Mr. R. S. ; Whipple, of Cambridge, the one ' now almost universally used on the ! Continent and in Great Britain and , America for scientific work. On this scale the freezing point of water is marked Odeg., and is called "zero," and the boiling point is marked lOOdeg., the interval (frequently referred to as "the fundamental interval") being divided into 100 equal parts. Mercury was. adopted at the outset as the thermometric substance, presumably because it was easier to distinguish than alcohol in the fine capillary tubing, and for nearly all work about atmospheric temperature it has maintained its position. This is not surprising when one considers over what a large range of temperature (.39deg. to 375deg.) it is liquid ; its. regularity of expansion fas compared with the gas thermometer) ,; the ease with which it can be obtained pure ; its pronerty of not wetting glass and its slow! vapour pressure. ■