THE ABBOT SILVER-DISK PYRHELIOMETER 



By L. B. ALDRICH 



Director, Astrophysical Observatory 



(With One Plate) 



DESCRIPTION OF INSTRUMENT 



In 1922 Dr. C, G. Abbot published a paper (The Silver-Disk Pyrhe- 

 liometer, Smithsonian Misc. Coll., vol. 56, No. 19) describing his 

 pyrheliometer, the method of use and possible errors, and listing the 

 constants of the various instruments. Since 1922 several modifications 

 of the instrument and of its method of use have been adopted, and 

 some 50 additional pyrheliometers have been prepared and sold to 

 interested institutions throughout the world. It therefore seems ad- 

 visable to bring Dr. Abbot's 1922 discussion up to date. 



Volume 2 of the Annals of the Astrophysical Observatory of the 

 Smithsonian Institution (p. 36) describes the original mercury pyr- 

 heliometer which Dr. Abbot built in 1902 and from which the silver- 

 disk pyrheliometer developed. In 1909 the form of instrument shown 

 in cross section in figure i was adopted. The following description of 

 it is taken from Dr. Abbot's paper (Smithsonian Misc. Coll., vol, 56, 

 No. 19) : 



The silver disk, a, shown in cross-section [fig. i], is bored radially with a 

 hole to admit the cylindrical bulb of a thermometer, b. 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 thermometer bulb. A soft cord soaked in shellac is 

 forced down at the mouth of the hole to prevent 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, b, 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 protects the thermometer. A slot 

 is cut in the right-hand side of the support tube throughout almost its whole 

 length to permit the reading of the thermometer. At the top of the support 

 tube a short piece is removable, in order that heat may be applied to the over- 

 flow bulb of the thermometer, to dislodge mercury which sometimes collects there 



1 Picein wax has been used in recent years. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 14 



