Emission and Transmission of Heat 



Fig. 183 



of the water vessel in the opposite opening, it is secured in the same 

 way by wooden wedges. 



Figure 183 shows the construction for pow- 

 dered material, the steam box is solid and sur- 

 rounded by a wooden frame to insulate it from 

 the tank, and the front of the frame is closed 

 by a thin sheet, either of glass or tin plate, cov- 

 ered on both sides with paper, the outer paper 

 serving to fasten the sheet in place. The pow- 

 dered matter is placed between the steam box 

 and the thin sheet. 



856. Figure 184 shows the thermopile and 

 the means of adjusting its position, a b cd is the 

 pile, e and /are two terminals in contact with the poles and to 

 which are connected the wires from the galvanometer. The ther- 

 mopile is held by two rods g h, fixed 

 to a frame ik, carrying a pinion, 

 turned by a key, engaging in the 

 rack Im. By this means the pile 

 can be adjusted so that the needle of 

 the galvanometer is at zero when 

 the two surfaces, radiating to the op- 

 posite faces of the pile, are the same 

 and at the same temperature. The rack is supported by the two 

 rods / n, terminating in the threaded rings q r, into which are 

 screwed cylinders which pass through the openings KK in the 

 tank (fig 178) and are fastened therein by slips of wood. 



857. Figure 185 represents the 

 arrangement used to determine the re- 

 lative radiating powers of different sur- 

 faces as needed for the purpose ex- 

 plained in paragraph 788. In the 

 openings L L L L, of the apparatus 

 just described, are placed two copper 

 vessels, one heated by steam, the other 

 containing water and so arranged that 

 the water can be heated to a suitable 

 degree; the inner faces of the vessels 

 are covered with the substances of which we wish to determine the 



p 



Fig. 184 



