GENERAL ACCOUNT OF RADIATION. 



221 



Hot 



is so adjusted that the liquid extends round the bend of the U and stands 

 at the same level on each side when the bulbs are at the same tempera- 

 ture. One of the bulbs is exposed to the radia- 

 tion to be measured, while the other is protected 

 from it, and the liquid is depressed by the 

 expansion of the air in the exposed bulb. 



Thermopile. Melloni, another pioneer, 

 used a thermopile, which is much more sensitive 

 than the differential thermometer. It consists 

 of a number of bars of bismuth and antimony 

 connected at the ends as represented in Fig. 128, 

 the bars being insulated from each other along 

 their sides. The beginning of the first bar and 

 the end of the last bar are connected to binding 

 screws, which in turn are connected to a galvano- 

 meter. If one set of alternate junctions is heated 

 by exposure to radiation, a current flows from 

 bismuth to antimony across the hot junctions, 

 proportional to the difference of temperature of 

 the hot and cold junctions, and the current is 

 indicated by the galvanometer. 



Thermopiles have been made containing 

 many hundred pairs of bars, the E.M.F. being 

 proportional to the number of junctions. But 

 the resistance increases with the number of 



bars and with a properly designed low -resistance galvanometer very 

 little is gained by this multiplication of bars. 



Bismuth \\ Antimony | 

 FIG; 128. Thermopile. 



Radio-Micrometer. This point is well 



brought out in the radio-micrometer, an 

 instrument first invented by D'Arsonval 

 and subsequently reinvented and made into 

 an instrument of extraordinary delicacy by 

 Boys.* It may be regarded as a combina- 

 tion of a thermopile of two bars and a 

 galvanometer, all in one. Fig. 129 shows 

 the principle of the instrument. The 

 lower ends of two short bars, respectively 

 of bismuth and antimony, are soldered to a 

 small copper disc Cu in the figure. To the 

 upper ends are soldered the ends of a copper 

 wire, completing the circuit and making it a 

 very narrow rectangle. This circuit depends 

 from a glass fibre ff, on which is a mirror m, 

 and this in turn from a quartz fibre qq. 

 The rectangle hangs between the two poles 

 of a strong permanent magnet. The whole 

 is enclosed in a protecting case of wood, 

 but there is a window for a beam of light to 



fall on m, and a tube is directed on to the disc Cu. If radiation passing 



through this tube warms the disc, it in turn warms the junctions with the 



* Phil. Trams., 1889, A., p. 158. 



S6 



FIG. 129. Kadio-micrometer. 



