THE SILVER DISK PYRHELIOMETER 



By C. G. abbot 

 Director, Astrophvsicai, Observatory of the Smithsonian Institution 



(With One Plate) 



In 1902 the writer designed a mercury pyrheliometer based on the 

 fundamental device of Pouillet, as modified by Tyndall. This in- 

 strument was described in the Annals of the Astrophysical Observa- 

 tory of the Smithsonian Institution, volume 2. As the measure- 

 ments have gone on at Washington, Mount Wilson, and Mount 

 Whitney for the determination of values of the solar constant of 

 radiation, various alterations have seemed desirable, which led in 

 1909 to the form of instrument here described. 



The silver disk, a, shown in cross-section, is bored radially with 

 a hole to admit the cylindrical bulb of a thermometer, h. The hole 

 in the disk has a thin lining of steel, so that a small quantity of 

 mercury may be introduced without alloying the silver, in order to 

 make a good heat conduction between the silver disk and the ther- 

 mometer bulb. A soft cord soaked in shellac is forced down at the 

 mouth of the hole to prevent the escape of mercury, and a ring of 

 Chatterton wax is sealed over the outside of the cord to make the 

 closure more perfect. 



The thermometer, h, is bent at a right angle, as shown, in order 

 to make the instrument more compact and less fragile. A nickeled 

 brass tube (shown partly cut away in the figure) supports and pro- 

 tects the thermometer. A slot is cut in the right-hand side of the 

 support tube throughout almost its whole length to permit the read- 

 ing of the thermometer. At the top of the support tube a short 

 piece is removable, in order that heat may be applied to the overflow 

 bulb of the thermometer, to dislodge mercury which sometimes col- 

 lects there during transportation. The thermometer is graduated 

 to tenths of degrees centigrade from — 15° C. to -j- 50° C. Two 

 points, 0° and -f 50°, are first marked on the stem by the makers, 

 and then the thermometer is graduated by equal linear intervals 

 without regard to the variations of cross-section of bore of the 

 stem. Before insertion in the instrument, a careful calibration of 

 the thermometer stem is made. 



Smithsonian Miscellaneous Ccllections Vol. 56, No. 19 



