NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 585 



the disc. The general arrangement will be seen in Fig. 6. The disc was 

 suspended by the four wires some distance below a small flat wooden box. 

 On the box was fastened a calorimeter can swathed in cotton and filled 

 with kerosene in which the constant thermo-junctions were immersed. 

 Copper wires soldered to the two ends of the thermo-electric series were 

 brought out of the calorimeter, and the circuit was closed through 1000 

 ohms in series with the 500 ohms resistance of galvanometer Gi. The 

 thermo-junctions in the disc were in series, and as each junction was mid- 

 way between the central plane of the disc and either face, it was assumed 

 that when the disc was slowly warmed by heating one face the electro- 

 motive forces obtained corresponded to the mean temperature of the 

 disc. One face of the disc was blackened by spraying it with powdered 

 lampblack in alcohol containing a trace of shellac. This method was 

 suggested by Prof. G. E. Hale and gives very fine and uniform dead 

 black coatings not inferior to good smoke deposits. 



For the energy measurements the bell-jar and the torsion balance were 

 removed from the platform P (Fig. 2) and a double walled copper vessel, 

 AB (Fig. 6), which served as a water jacket surrounding a small air 

 chamber O, was mounted in the same place. A tube 2 cm. in diameter 

 was soldered into the front face of the jacket to admit the light beam into 

 the chamber C. This opening was covered by a piece of plate glass 

 similar to the plates forming the larger windows in the bell-jar. 



The needle system in Gi, a four-coil du Bois Rubens galvanometer, 

 was suspended in a strong magnetic field so that its period was about 

 four seconds. The system was heavily damped by a mica air-fan of large 

 surface. The disc junctions and galvanometer responded quickly to the 

 radiation, as was shown by the reversal of motion of the magnet system 

 1.2 seconds after the light was cut off from the disc when the latter was 

 a few degrees above the temperature of the room. 



The disc was calibrated for temperature in terms of the deflection for 

 a definite sensitiveness of the galvanometer Gy. For this purpose the 

 disc was immersed in a kerosene bath and the galvanometer deflection 

 measured for two different temperatures of the disc. One of these was 

 ah lut 18'^ C. above the comparatively steady temperature of the room, or 

 calorimeter containing the standard temperature junctions (see Fig. fi), 

 and the other about the same number of degrees below the room temper- 

 ature. These two temperatures were measured by a Fuess Standard 

 Thermometer divided into tenths of a decree and calibrated at the 

 Reichsanstalt. Two calibrations of the silver disc were made some days 

 apart. One of these series appears in full in Table V. The first three 



