Page:The American Cyclopædia (1879) Volume XV.djvu/728

 698 THERMOMETER silver, copper, zinc, iron, antimony. The di- rection of the current often changes when the couple is heated beyond a certain degree. Thus, in a copper and iron circuit, the current passes from the copper to the iron through the heated part when the temperature is not higher than 570 ; above this the curent passes in the opposite direction. The cause of ther- mo-electric currents is diversity in the molec- ular structure of the elements, and Becquerel ascribes them to unequal propagation of heat in the different parts of the circuit. A thermo- electric pile, or battery, in which a series of several couples are joined somewhat like the arrangement in a voltaic pile, or at least with the opposite poles of the elements in contact with each other, was devised by Nobili. A modification of this is shown in fig. 2, in which the lowest plate is bis- muth, the next above antimony, the next again bismuth, and so on, the last plate being antimo- ny. These sets of ele- ments are arranged in a copper frame, P, in four vertical series, making FlG< 2. ia all 20 couples. The terminal plates are con- nected with binding screws, m and w, by which they may be connected with a resis- tance measurer or rheostat, or with a sine or a tangent galvanometer. (See GALVANISM, vol. vii., pp. 593-'5, and DIATHERMANCY, vol. vi., p. 81.) When the pile is composed of a great number of pairs and connected with a very delicate galvanometer, it may be used to detect the slightest changes of temperature; it is much employed in physical investigations, and will undoubtedly in time have extended practical use in physiology and medicine. THERMOMETER (Gr. Oepfirj, heat, and pirpov, a measure), an instrument to measure tempera- tures. It is formed of two or more different substances, the volumes of which expand and contract to different extents when they are simultaneously exposed to the same differences in intensity of heat. The first attempt at indi- cating to the eye differences of temperature seems to have been by the contrivance vari- ously ascribed to Drebbel of Holland and Sanc- torius of Italy, about the beginning of the 17th century, and known as a weather glass. This was very rude and inaccurate, consisting of a glass bulb and tube inverted, opening be- low into a cup of colored liquid, which, the air of the bulb having been partly expelled by heat, rose in the tube, and stood at different heights according as the air remaining in the bulb was more or less expanded by heat. This, the origin of the common air thermometer, as improved by Boyle and by the Florentine academicians, became transformed to a smaller bulb with upright stem of somewhat fine bore, the contained liquid being colored spirits of wine ; boiling this to expel air, the tube was hermetically sealed, and the whole then affixed to a case. A scale of degrees was also intro- duced, its fixed points being the cold of snow or ice and the greatest heat known at Flor- ence ; it was of necessity very variable in its indications. At this stage in the progress of thermometry, much discussion in regard to the most suitable fixed points for the scale, the best substance for use in the instrument, &c., including that of the question whether water did not freeze at different temperatures in different latitudes, was carried on in Eng- land and on the continent. Hooke advocated as the lower fixed point the temperature of freezing water. Newton seems first to have discovered or taken advantage of the facts, that a thermometer placed in melting snow or ice always indicates the same temperature, and always very nearly one temperature in boiling water ; but of oil, which he suggested for the liquid in the bulb, the movements were found to be too sluggish and uncertain. Romer, overcoming a prejudice that seems to have ex- isted in regard to unequal expansion of mer- cury, first adopted that liquid ; and he doubt- less devised the instrument and scale usually attributed to Fahrenheit of Amsterdam (1720), the latter constructing and introducing the in- strument, so that it became generally known throughout Europe in the first half of the 18th century. Of this thermometer, the lower fixed point, or zero, was taken at 32 below freezing point of water ; but whether as the cold ob- tained by its maker by mixing salt and snow, or as the greatest cold observed in Iceland, and in either case as the supposed point of absolute cold, is not now definitely known ; and since Fahrenheit kept his graduation of thermome- ters a secret, the same must be said respecting the choice of a scale of 180 between the fixed points. Celsius of Sweden (1742) introduced a scale of 100 between the fixed points ; this was adopted in France at the time of the revolution, and named the thermometre centi- grade; and owing to its convenient decimal division, it has been wholly adopted in sev- eral countries of Europe, while it is coming into general use among scientific men through- out the world. For the general principles upon which the use of the thermometer de- pends, see EXPANSION, HEAT, and PYROMETER. An increase in the temperature of a body is generally accompanied by an increase in its volume, and a decrease in its temperature by a contraction in its volume. Definite changes in the volume of a given substance may be used as indications of this substance having different definite temperatures, and this substance will have the temperature of the bodies by which it is surrounded, or of the medium in which it is immersed, and thus serve to measure their temperature. The substances generally used in the thermometer are glass and mercury, and the observed change of volume is the difference in the ehange of volume of the glass and of