Page:Collier's New Encyclopedia v. 09.djvu/411

THERMOELECTRICITY 3i9 THEROID ments. But as the electric current de- veloped in a single pair is very weak, a considerable number are usually com- bined to form a thermoelectric pile or battery. Bismuth and antimony are the m.etals usually employed, the difference in electromotive forces being greater be- tween them than between any other two metals conveniently obtainable. THERMOGRAPH, a thermometer pro- vided with a recording device. The form generally used has a crescent shaped bulb, filled with alcohol and hermetically sealed. A change in the temperature af- fects the curve of the bulb, and the al- teration of form is communicated to a series of multiplying levers, which ^ in turn exaggerate the motion, and give im- petus to a recording pen. This pen makes a tracing on the ruled paper of a moving cylinder. Bartlett's thermo- graph is especially designed for green- houses, having a register for indicating in the office tho 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. THERMOMETER, in physics, an in- strument for measuring intensity of heat or temperature, by means of expansion of a liquid or gas. Mercury is generally employed, and an ordinary thermometer 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 expan- sion of the mercury. A rise of tempera- ture is indicated by a rise of the mercury in the tube, owing to expansion and, conversely, a fall of temperature is in- 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 fixed points. On the Centigrade or Cel- sius thermometer (used by scientific men all over the world and in general use on the continent of Europe), the distance between the two points is divided into 100 degrees, the freezing point being 0°, and the boiling point being 100°; on the Reaumur thermometer (used only in N. W. Europe), the distance is divided into 80 degrees, the freezing point being 0° and the boiling point 80° ; on the Fahren- heit thermometer (in general use in the w United States and England), the distance is divided into 180 degrees, but, since zero is 32 degrees below the freezing point, the freezing point is 32°, and the boiling point is 212°, Degrees above 0° are termed -(- degrees, while those below 0° are termed — degrees. C. -H 5X9 + 32 = F. I P. — 32-T-9X4 = B. R. -^4X94-32 = F. C. -h6X4 = R. F. — 82^9X 6 = C. I R. -j-4X5=:C, Mercury can only be used for tempera- tures between — 40° and + 675°, since it freezes at 40° and boils at 675°. For lower temperatures alcohol is used; and for high temperatures air thermometers are employed, in which changes of ten> perature are measured by the expansion or contraction of a known volume of air. In deep-sea thermometers, used for as- certaining the temperature of the sea, the bulb is specially protected against the pressure of the water. THERMOPYL.ffi, a famous pass lead- ing from Thessaly into Locris, and the only road by which an invading army can penetrate from Northern into Southern Greece. It lies S. of the present course of the river Spercheius, between Mount (Eta and what was anciently an impassa- ble morass bordering on the Maliac Gulf. In the pass are several hot springs, from which Thermopylas probably received the first part of its name. Thermopylae has won an eternal celebrity as the scene of the heroic death of Leonidas (q. v.) and his 300 Spartans in their attempt to stem the tide of Persian invasion (480 B. c). Again in 279 b. C. Brennus, at the head of a Gallic host, succeeded, through the same treachery that had se- cured a victory to Xerxes, in forcing the united Greeks to withdraw from the pass. THERMOPYLAE OP AMERICA, a phrase applied to Fort Alamo, Tex.; in allusion to the heroic defense of it, made in 1836, by a small body of Texans against a force of Mexicans 10 times their number. During the subsequent struggle for independence the Texan war cry was "Remember the Alamo!" the Mexicans having murdered the six de- fenders of the fort whom they found alive on its surrender. THERMOSCOPE, an instrument for indicating relative differences of tem- perature. The term was applied by Count Rumford to an instrument invent- ed by him, and similar in principle to the differential thermometer of Profes- sor Leslie. THEROID, animal ; having animal pro- pensities or characteristics; specifically applied to idiots, who in habits or appear- ance resemble any of the lower animals. Cyc. Vol. IX