THERMOELECTRICITY 



5786 



THERMOMETER 



beds of uncooled lava or by coming in contact 

 with hot rocks. Hot springs in nonvolcanic 

 regions are probably caused by the circulation 

 of water at great depths in the earth before it 

 reaches the surface. Many mineral springs are 

 hot springs. See MINERAL WATERS; SPRING. 



THERMOELECTRICITY, ther'mo e lek tris'i 

 ti, the electric current set up in a circuit of two 

 or more unlike metals, or in a circuit of one 

 metal the different sections of which are in 

 unlike physical states, when one of the points 

 of contact is heated or cooled. As electrical 

 energy may be transformed into heat and light, 

 so heat and light may be changed into electrical 

 energy. This may be shown by connecting an 

 iron wire with a German-silver wire and attach- 

 ing the loose ends to a galvanometer. When 

 the joined ends are heated or coojed, the needle 

 will show a responsive movement. If bismuth 

 and antimony be substituted for iron and Ger- 

 man-silver, the effect will be still more marked. 

 Since their electromotive force is greater than 

 that of any other two readily obtainable metals, 

 antimony and bismuth are used in constructing 

 the thermoelectric pile. 



This pile, or battery, is constructed of alter- 

 nate bars of antimony and bismuth soldered 

 together, and is so sensitive to changes in tem- 

 perature that it makes a very excellent substi- 

 tute for a thermometer. As long as both faces 

 have the same temperature, there is no current, 

 but the slightest variation sets up a flow of 

 electricity. This change may be almost in- 

 finitely small. There is, for example, enough 

 heat in the body of a fly walking across one 

 face of the pile to deflect the needle of the 

 galvanometer. It is obvious that such an in- 

 strument is useful for detecting the most mi- 

 nute changes in temperature. This peculiar 

 action of metals of different heat-conducting 

 powers in starting electric currents was first ob- 

 served by Seebeck in 1822. 



See ELECTRICITY,, and the various related sub- 

 jects there named. 



THERMOGRAPH, ther'mo graf, an instru- 

 ment which automatically makes a record of 

 changes of temperature. A standard pattern, 

 shown iu the accompanying illustration, is pro- 

 vided with a spiral coil of metal which unwinds 

 with a rise of temperature and winds up when 

 cooled. These movements are recorded by a 

 long needle carrying a pen, which traces a cor- 

 responding zigzag line on a revolving paper 

 sheet moved by clockwork. Degrees of tem- 

 perature are marked off on this sheet by ver- 

 tical divisions, while hours of time are desig- 



nated by horizontal distances. Usually the 

 paper is prepared to receive the record of one 

 week, and when the period is completed the 



THE THERMOGRAPH 



sheet is removed and a fresh one put in its 

 place. 



THERMOMETER, ther mom' e ter, an in- 

 genious instrument for indicating temperature, 

 one of the most useful applications of the 

 law that bodies expand on heating and contract 

 on cooling. There are various forms of ther- 

 mometer, but the one in most common use con- 

 sists of a glass tube, with a bulb of mercury or 

 alcohol at the lower end. An increase in tem- 

 perature causes the liquid to rise in the tube, 

 and the point to which it rises is shown on a 

 scale placed on or back of the tube. As the 

 liquid in the tube has practically the same 

 temperature as that of the surrounding air, the 

 degree of temperature can be accurately read 

 by noting the height to which the mercury or 

 alcohol rises. There are three standard scales 

 the Fahrenheit, Centigrade and Reaumur but 

 only the first two are in general use. The 

 Fahrenheit (abbreviated F.) scale has 180 divi- 

 sions, extending between 212 and 32 above 

 zero. These limits are respectively the boiling 

 and freezing p'oints of water. On the Centi- 

 grade (C.) the freezing point is marked 0, and 

 the boiling point 100. This scale is used very 

 generally in scientific calculations. The Reau- 

 mur scale has for the freezing point and 80 

 for the boiling. For rules for changing Centi- 

 grade readings into Fahrenheit and vice versa, 

 see CENTIGRADE. 



How a Thermometer Is Made. The glass 

 tubes used for thermometers are constructed so 

 that expansion and contraction will be equal 

 throughout the length, and before being ac- 

 cepted they are thoroughly tested. When a 

 tube is found to be satisfactory, a bulb is blown 

 at one end of it, and the opposite end is worked 

 into a funnel shape. A little mercury is in- 



