Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/594

560 HEAT pending on difference of temperature between the two places, and showing zero continuously when the tempera tures of the two places are varied, provided they are kept exactly equal. Every kind of differential thermoscope, and Con- of continuous intrinsic thermoscope, must be founded on tinuous some property of matter, continuously varying with the thermo- temperature, as density of a fluid under constant pressure, ?es * pressure of a fluid in constant volume, volume of the liquid part of a whole mass of liquid and solid kept in constant volume, 1 steam-pressure of a solid or liquid, 2 shape or density of an elastic solid under constant stress, stress of an elastic solid in a constant state of strain, viscosity of a fluid, electric current in a circuit of two metals with their junctions at unequal temperatures, electric resistance of a conductor, magnetic moment of a steel or loadstone magnet. Examples : (1) Leslie s differential air thermometer; (2) steam- pressure differential thermometers ( 39-44 below) ; (3) Joule s hydraulic and pneumatic differential thermoscopes (Memoirs Chcm. Soc. , vol. iii. p. 201 ; Proc. Lit. and Phil. Soc. Manchester, vol. iii. p. 73 ; Ibid., vol. vii. p. 35); (4) viscosity differential thermoscope (Proceedings .S.E.,Aiml 5, 1880) ; (5) thermo-electric differential thermometer ; (6) Siemens electric resistance differential thermo meter ; (7) thermo-magnetic differential thermometer (see Proceed ings R.S.E. for April 5, 1880). 14. Intrinsic Thermoscopes. An intrinsic thermoscope is an instrument capable of indicating one definite tempera ture or several definite temperatures, or all temperatures within the range of the instrument, whatever it may be the temperature or temperatures indicated being intrinsically determined by the constitution of the instrument and indi cated by some recognizable feature of the instrument which changes discontinuously or continuously, as the case may be, and which is always the same when the instrument is brought back again and again to the same temperature, whatever changes it may have experienced in the intervals. Discon tinuous intrinsic thermo.scopes show only a limited number of temperatures. A continuous intrinsic thermoscope shows any temperature whatever throughout the range of efficiency of the instrument, ideally any temperature whatever, though in practice every thermoscope is limited, some with both inferior and superior limit, as the mercury thermometer by the freezing of mercury at about - 39 C., and the bursting pressure of mercury-steam a little above + 300 C. ; others with only a superior limit, as metallic thermoscopes, whether thermo-elastic, or thermo-electric, or electric-resistance, or thermo-magnetic, by the melting of their substances at very high temperatures, or, in the case of the thermo-magnetic instrument, by the total or partial loss of its magnetism at some temperature much below the melting point of its sub stance. A continuous intrinsic thermoscope, when applied to a body whose temperature is changing, shows continuously every variation of temperature within its range of efficiency. 15. Discontinuous Intrinsic Thermoscopes. A single intrinsic thermoscope is a thermoscope which shows whether tha temperature of the body to which it is applied is higher or lower than some one definito temperature depending on the intrinsic quality of the instrument. Examples: (1) a piece of ice, or of wax, or of fusible metal ; (2) an apparatus for boiling water or other liquid under a perfectly constant pressure ; (3) an apparatus for boiling water under the natural atmospheric pressure, and a barometer to measure exactly what the pressure is at the time. A multiple intrinsic thermoscope might be made by pre paring a graduated series of metallic alloys, num.bering them in order of their melting points, and arranging them together conveniently for use. The temperature might be reckoned numerically, according to the number of the alloys that melt, when the whole series is exposed to the tempera ture to be tested. This discontinuous numerical reckoning of temperature is perfectly analogous to the Birmingham 1 This is the principle of the ordinary mercury or spirit thermometer. 3 For definition of steam see 17 below. reckoning of wires and sheet metals by numbered gauges. Ideally it may be made infinitely nearly continuous by making a series of alloys with fine enough gradation of com position, but the method is in its essence discontinuous. It is useful for many special applications in science and in the arts, as for instance in that very fundamental one ( 12) of giving one of the fixed points in the ordinary thermometric scale, the " freezing point " ; also in a form of safety valve for boilers or hot-water pipes, in which a plug fixed by solder is released by the melting of the solder when the temperature reaches a certain limit ; also an exceedingly useful guard against overheating in the flue of a stove, by which a stopper is allowed to fall by the melting of a leaden support, and stop the draught, before the tem perature reaches the highest limit judged permissible. 16. Continuous Intrinsic Thermoscopes. Continuity of indication requires, as said above ( 13), choice of some pro perty or properties of matter varying continuously with tem perature, such as those enumerated in 13. A continuous intrinsic thermoscope must have a feature, depending on the chosen property of matter, which shall vary with perfect continuity when the temperature is gradually changed, and shall always be the same when the instrument is brought to the same temperature again and again, whatever varia tion of temperature it may have experienced in the intervals. The accuracy of an intrinsic thermometer, whether discon tinuous or continuous, depends upon permanence of quality of the material and of the mechanical constitution of the instrument, according to which the recognized feature shall always be very accurately the same for the same temperature. The sensibility or delicacy of a continuous intrinsic thermo meter depends upon the recognizability of change in its indicating feature with very small change of temperature. 17. The property of matter chosen as the foundation of almost all ordinary continuous intrinsic thermoscopes in common use is interdependence of the density, the tempera ture, and the pressure of a fluid. The only other thermc- scopes which can be said to be in common use at all are " metallic thermometers " (see THERMOMETER ; MATTER, PROPERTIES OF); these depend upon the change of shape of a rigid elastic solid under a stated stress, or on the change of shape of a compound solid, composed of two elastic solids of different substances melted or soldered together. For the present we confine our attention to the former and much larger class of instruments. The general type of all those instruments, except the steam-pressure thermometer ( 39-46 below), is a glass measure, measur ing the bulk of a fluid. To give the requisite practical sensibility to the measurement, the glass, except for the case of the constant-pressure gas thermometer ( 64-67 below) and of the steam-pressure thermometer, i s made of a shape which may be generally described as a bottle with a long narrow neck. The body of the bottle, which may either be spherical or of an elongated form, is called the bulb, and the neck is called the tube or stem (stem we shall most frequently call it, to obviate ambiguities without circumlocutions). The thermometric fluid may be all liquid, as mercury, or oil, or alcohol, or ether, or glycerine and water; or it may be all gas, as common air, or hydrogen, or carbonic acid ; or it may be partly liquid and partly steam (steam being a name which we Stc shall invariably use to designate the less dense portion t1fcl of a fluid substance at one temperature and pressure tl( throughout, and in equilibrium, with two parts of different densities). This last case is different from the two pre ceding, in respect to the character of the thermometric indication : the whole volume of the thermometric sub stance may be changed from that of all liquid to that of all steam without changing the temperature or the pressure,

and the pressure cannot be changed without changing the