Thermoelectricity
electricity produced at the junction of two metals, or at a point where a molecular change occurs in a bar of the same metal, when the junction or point is heated above or cooled below the general temperature of the conductor. Thus when wires or bars of metal or different kinds, as bismuth and antimony, are placed in close contact, end to end, and disposed so as to form a periphery or continuous circuit, and heat then applied to the ends or junctions of the bars, electric currents are produced. The thermoelectric battery, or pile, an apparatus much used in delicate experiments with radiant heat, consists of a series of little bars of antimony and bismuth (or any other two metals of different heat-conducting power), having their ends soldered together and arranged in a compact form; the opposite end of the pile being connected with a galvanometer, which is very sensibly affected by the electric current induced in the system of bars when exposed to the slightest variations of temperature.
To the combined arrangement of pile and galvanometer the name of thermomultiplier is given. Two metal bars of different heat conducting power having their ends soldered together, and the combined bar then usually bent into a more or less horseshoe or magnet form for the purpose of bringing their free ends within a conveniently short distance, designated a thermoelectric pair, are much used in thermoelectric experi- THERMOELECTRICITY ments. But as the electric current developed in a single pair is very weak, a considerable number are usually combined to form a thermoelectric pile or battery. Bismuth and antimony are the metals usually employed, the difference in electromotive forces being greater between them than between any other two metals conveniently obtainable. vided with a recording device. The form generally used has a crescent shaped fects the curve of the bulb, and the alsealed. A change in the temperature affects the curve of the bulb, and the alteration of form is communicated to a series of multiplying levers, which in turn exaggerate the motion, and give immakes a tracing on the ruled paper of a makes a tracing on the ruled paper of a moving cylinder. graph is especially designed for green- Bartlett's thermohouses, having a register for indicating in the office the temperatures prevailing in the different houses. This apparatus is sometimes arranged so that a rise in temperature to 100° will ring a gong.
Thus arranged it serves as a warning of fire. strument for measuring intensity of heat THERMOMETER, in physics, an instrument for measuring intensity of heat or temperature, by means of expansion of a liquid or gas. Mercury is generally employed, and an or consists of a spherical or cylindrical glass bulb at the end of a very fine tube, the bulb being completely filled and the tube partly filled, with mercury, while the space above the mercury contains only a small quantity of mercury vapor, which offers no resistance to the expansion of the mercury. A rise of temperature is indicated by a rise of the mercury in the tube, owing to expansion and, conversely, a fall of temperature is in- in the tube, owing to expansion and, dicated by a fall of the mercury in the tube. A graduated scale is attached with two fixed points: the lower, or freezing point, and the upper, or boiling point, of water. The distance between the two fixed points is then divided into a certain number of equal parts, or degrees, which are continued above and below the two sius thermometer (used by scientific men all over the world and in general use ordinary thermometer on the continent of Europe), the distance between the two points is divided into and the boiling point being 100°; on the W. Europe), the distance is divided into 80 degrees, the freezing point being 0° W. Europe), the distance is divided into and the boiling point 80°; on the Fahrenheit thermometer (in general use in the THEROID United States and England), the distance is divided into 180 degrees, but, since zero is 32 degrees below the freezing point is 32°, and the boiling point is 212. Degrees above 0° 0° are termed + degrees, while those below C.5×9+ 32 = F.
49+ 32 = F.
F.-329 × 5 = C.
R. degrees.
F.-329 X 4 = R. 5X4R. 4×5= C.
C.
R. tures between it freezes at 40° and boils at 675°. For Mercury can only be used for temperalower temperatures alcohol is used; and 40° and +675°, since for high temperatures air thermometers perature are measured by the expansion are employed, in which changes of tem- In deep-sea thermometers, used for asor contraction of a known volume of air. certaining the temperature of the sea, the bulb is specially protected against the pressure of the water. ing from Thessaly into Locris, and the