222 



HEAT. 



bismuth and antimony bars, and the same E.M.F. is produced as if the 

 bars were directly joined and raised to the same temperature. A current 

 flows counter-clockwise round the circuit as represented, and it tends to 

 turn at right angles to the plane of the paper, and the torsion of the fibre, 

 being the only resistance to turning, the instrument may be made ex- 

 ceedingly sensitive. 



Bolometer. In 1881 Langley * described an instrument which he 

 named the bolometer, which is probably of the same order of sensitive- 

 ness as the radio-micrometer, but has the advantage of portability and 

 capacity for receiving radiation in any direction. The bolometer is a 

 Wheatstone Bridge and may be represented diagrammatically by Fig. 

 130. AB is a thin flat strip of metal in the original instrument a strip 



of iron 7 mm. long, -177 mm. broad, 

 and '004 mm. thick. It had a re- 

 sistance of 0'9 ohm. CD is a wire oi' 

 equal resistance. EF and GH are the 

 ratio arms, here of course nearly equal, 

 and capable of adjustment to give a 

 balance in the galvanometer. The 

 bridge is enclosed in a protecting case, 

 but opposite AB is a shutter which 

 can be opened to admit radiation. 

 When AB receives radiation which 

 warms it, its resistance rises and the 

 balance in the bridge is destroyed. 

 The current through the galvanometer 

 is proportional to the rise in tem- 

 perature of AB. The instrument is 

 variously modified according to the pur- 

 pose for which it is used, but it still re- 

 tains its simplicity of principle. By its 

 aid Langley has greatly extended our 

 knowledge of radiation, and it is gener- 

 ally used by other workers, having 

 largely displaced the thermopile in investigations on radiant energy. 



Radiant Energy and Light resemble each Other. Quite com- 

 mon experience tells us that radiant energy has many properties in 

 common with light, with which it is so frequently associated, as with 

 sunlight, firelight, and so on. It is propagated in straight lines, and 

 forms shadows as we know when we cross from the shady side of a 

 street on a winter's day, or when we use a fire-screen to keep off the heat 

 as well as the light of a fire. It is also reflected and refracted with 

 light, as we know by the charring of a piece of paper or wood placed in 

 the focus of the sun's light formed by either a mirror or a " burning " 

 lens. We may also reflect the dark radiation from a heated ball by a 

 mirror, just as we reflect light. It further resembles light in travelling 

 with great rapidity, for the obstruction of the light of the sun by the 

 most distant cloud is accompanied by a diminution of the warmth 

 received from him ; and in eclipses by the moon there is a marked fall of 

 temperature as well as a loss of light. 



* Nature, vol. xxv., 1881, p. 14. For the construction of bolometers we may refer to 

 Kurlbaum, Wied. Ann., xlvi. p. 204, 1892; or Langley, Am. Journ. Sci., v. p. 241, 1898. 



FIG. 130. Bolometer. 



