For the measurement of modern temperatures there arc two standard thermometers: the Fahrenheit used in this country and England for ordinary purposes, and the Centigrade used in Continental countries, and by scientists.
A thermometer consists of a cylindrical glass tube of a uniform bore and diameter, sealed at one end. A fluid is first placed in the tube, which then is heated until the fluid expands and fills the tube, thereby driving out the air. It is necessary to create a vacuum; otherwise the air would prevent the fluid from expanding in the closed tube. After the air has been driven out the tube is sealed. It is then placed in an atmosphere of free steam representing the boiling point of water, and next in an ice bath consisting of broken pieces of ice floating in water. The positions of the liquid at both of these points are marked on the tube, the boiling point representing 212° F. and the freezing point 32° F. The intervening distance between these two points is divided into 180 divisions and each division is called a degree. The Centigrade thermometer has 100 divisions between these two points. Mercury is especially adapted for use in thermometers on account of the uniformity with which it increases in volume, and also on account of its extremely high boiling point. Alcohol colored with some dyestuff is used in cheap household thermometers.