SMITHSONIAN MISCELLANEOUS COLLECTIONSVOLUME 111, NUMBER 14
^obsfeins JfunbTHE ABBOT SILVER-DISKPYRHELIOMETER(With One Plate)
BYL. B. ALDRICHDirector, Astrophysical ObservatoryM§^m:(Publication 3991)
CITY OF WASHINGTONPUBLISHED BY THE SMITHSONIAN INSTITUTIONDECEMBER 8, 1949
Zi^t JSort (g>afttmore fpueeBALTIMORE, MB., D. S. A.
THE ABBOT SILVER-DISK PYRHELIOMETERBy L. B. ALDRICHDirector, Astrophysical Observatory(With One Plate)DESCRIPTION OF INSTRUMENTIn 1922 Dr. C, G. Abbot published a paper (The Silver-Disk Pyrhe-liometer, Smithsonian Misc. Coll., vol. 56, No. 19) describing hispyrheliometer, the method of use and possible errors, and listing theconstants of the various instruments. Since 1922 several modificationsof the instrument and of its method of use have been adopted, andsome 50 additional pyrheliometers have been prepared and sold tointerested 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 theSmithsonian 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 shownin cross section in figure i was adopted. The following description ofit 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 ahole to admit the cylindrical bulb of a thermometer, b. The hole in the disk hasa thin lining of steel, so that a small quantity of mercury may be introducedwithout alloying the silver, in order to make a good heat conduction betweenthe silver disk and the thermometer bulb. A soft cord soaked in shellac isforced down at the mouth of the hole to prevent escape of mercury, and a ringof Chatterton ^ wax is sealed over the outside of the cord to make the closuremore perfect.The thermometer, b, is bent at a right angle, as shown, in order to makethe instrument more compact and less fragile. A nickeled brass tube (shownpartly cut away in the figure) supports and protects the thermometer. A slotis cut in the right-hand side of the support tube throughout almost its wholelength to permit the reading of the thermometer. At the top of the supporttube 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 there1 Picein wax has been used in recent years.SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 14
SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, III
Fig. I.—Abbot silver-disk pyrheliometer.
NO, 14 ABBOT SILVER-DISK PYRHELIQMETER ALDRICH 3during transportation. The thermometer is graduated to tenths of degreescentigrade from —15° C. to +50° C. Two points, 0° and +50°, are firstmarked on the stem by the makers, and then the thermometer is graduatedby equal linear intervals without regard to the variations of cross-section ofbore of the stem. Before insertion in the instrument, a careful calibrationof the thermometer stem is made.The silver disk, a, is enclosed by a copper cylindrical box, c, halved togetherfor convenience in construction. Three small steel wires, not shown in thefigure, support the silver disk. These wires lie in the plane of the center ofthe disk at 120° intervals apart. Midway between them are three brass screws,not shown, which may be screwed through the walls of the box, c, up to theirheads. These screws in that position clamp the silver disk tightly. Theirpurpose is to prevent the breakage of the thermometer if jarred during trans-portation. These screws must be loosened during observations.The copper box, c, is enclosed by a wooden box, d, to protect the instru-ment from temperature changes. This box is also halved together and fas-tened by long wood-screws, one of which is seen near the letter, d, in the figure.Sunlight may be admitted through the tube, e. This tube is provided witha number of diaphragms, /i, ji, /a, having circular apertures. The aperture, jz,nearest the silver disk is slightly smaller than the others, and slightly smallerthan the disk itself. Thus it limits the cross-section of the sunbeam whoseintensity is to be measured. The entire interior of the tube, e, the box, c,and the silver disk, a, are painted dead black with lamp-black mixed in alcohol,with a little shellac added to cause the lamp-black to stick. To secure a fine,even coat, the mixture is filtered through cheesecloth before applying, and onthe disk it is warmed with an alcohol lamp until the brush marks disappear.A rotatable shutter, g, with three nickeled parallel metal plates, h h h, is pro-vided for cutting off the sunlight as desired. The top of the tube, e, carriesa screen, k, large enough to shade the wooden box, d. This screen also sup-ports the thermometer tube, and the axis of the exposing shutter just men-tioned. A small hole in the part which supports the thermometer admits alittle guiding beam of sunlight, i, whose use is to assure the observer that theinstrument points toward the sun.The pyrheliometer is carried upon an equatorial stand, as shown in the figure.A worm and wheel mechanism is provided for following the sun. No clock-work is needed, as it is sufficient for the observer to move the worm slightlytwo or three times a minute.In 1927 two alterations were adopted: (i) the tube e was length-ened and (2) the base was enlarged to counteract top-heaviness dueto the longer tube. These changes are shown in the photograph, plateI. The short tube form of 1909 when the shutter was opened exposedeach point on the silver disk to a cone of sky io°38' in diameter. Sincethe sun subtends a diameter less than i/20th as great, the sky areaexposed is at least 400 times that occupied by the sun. In very clearskies the radiation from this area of sky is negligible as compared tothat from the sun. But measurement at Mount Harqua Hala in 1925 ^2 Annals of the Astrophysical Observatory of the Smithsonian Institution,vol. 5, p. 83.
4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Illin a very hazy sky indicated that the readings were as much as 2-^percent too high. To diminish the error from this source all silver-disk pyrheliometers made after 1926 carried tubes 32 cm. long inplace of the original 15 cm. The exposed sky area was thus reducedfrom .0043 hemisphere to .0013. As opportunity has arisen the fol-lowing older instruments have been similarly modernized by substi-tuting the longer tube and enlarging the base: S.I. Nos. i, 5, 16, 17,26, 31, 41, 42, and 44. In hazy skies the error due to sky radiationnow seldom exceeds ^ percent.While it is possible, from a study of the dimensions and physicalproperties of the instrument, to reduce its readings to heat units,there are uncertainties in thus using it as an absolute standard. Wehave preferred to consider it as a secondary instrument, the constantof each individual pyrheliometer being determined by careful com-parisons with Smithsonian standard instruments.^ Readings of thesilver-disk instrument are proportional to the intensity of radiationof the sun, and are comparable one with another at all times andplaces. Multiplying the corrected readings by the constant of the in-strument reduces them to true heat units.DIRECTIONS FOR USEThese directions, with minor alterations to conform with our pres-ent practice, are taken from Dr. Abbot's paper, mentioned above.SETTING UP1. If the mercury column of the thermometer is broken, remove thelittle screw at the side near the upper end of the nickel-plated tube,and take off the upper portion of the tube. Then heat the exposedstem cavity of the thermometer gently in a smoky flame (a matchflame is good) until the mercury is expelled from the cavity. Then,holding the thermometer vertical, shake the instrument repeatedlywith a downward jerk until the mercury columns join.2. Remove the two little ivory or fiber plugs (using pliers if nec-essary) and unscrew the two brass screws under them, and also un-screw the third similar screw seen through the trunnion on the otherside of the case. About three complete turns of each screw is proper.Insert the two ivory plugs. When packing the instrument again for3 Descriptions of Smithsonian absolute standard pyrheliometers are given inAnnals of the Astrophysical Observatory of the Smithsonian Institution, vol.3, p. 52; vol. 6, p. 5; Smithsonian Alisc. Coll., vol. 87, No. 15; vol. 92, No.13; vol. no, No. 5.
NO. 14 ABBOT SILVER-DISK PYRHELIOMETER—ALDRICH 5a journey, screw in the three scretvs as far as they mill go. Their pur-pose is to clamp the silver disk to protect the thermometer duringtransportation.3. Unscrew the two pivots from the sides of the ring of the mount-ing, insert the pyrheliometer so that the thermometer is not next tothe worm wheel, and screw in the pivots.4. Unclamp the half ring and set the polar axis approximately forthe latitude of the place. The thermometer should be next to the upperend of the axis. ADJUSTMENTS5. For quick adjustment in right ascension guide the pyrheliom-eter with one hand and loosen the lower right-hand milled screw (asseen from the upper end of the polar axis). The worm may then belowered out of engagement with the wheel and the change made.6. To follow the sun, adjust in right ascension and declination untilthe sun shining through the little hole in the upper plate forms itsimage on the scratched spot on the nickeled piece below. When ex-posing to solar radiation rotate the worm screw a little (about onceevery half minute) to follow the sun.7. When about to observe, push aside the cover, leaving only theshutter to shade the silver disk. \Mien through with each series ofreadings, close the cover to keep out dust.OBSERVATIONS8. When reading the thermometer the observer should hold his headso that the reflection of each dark line of the scale near the degree tobe observed, as seen in the mercury thread, is coincident with thecorresponding dark line. This prevents parallactic errors of reading.9. Having adjusted the instrument to point at the sun and openedthe cover, read the thermometer exactly at 20 seconds after the be-ginning of the first minute. Read again after lOO seconds, or at thebeginning of the third minute, and immediately after reading openthe shutter to expose to the sun. Note that the instrument is then cor-rectly pointed. After 20 seconds read again. After 100 seconds more(during which the pointing is corrected frequently), or at the begin-ning of the fifth minute read again, and immediately close the shutter.After 20 seconds read again. After 100 seconds read again, or atthe beginning of the seventh minute. Continue the readings in theabove order, as long as desired. Readings should be made within •§•second of the prescribed time. Hold the watch directly opposite thedegree to be observed, and close to the thermometer. Read the hun-dredths of degrees first, the degree itself afterward.
6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IllID. For results not requiring the highest accuracy the above de-scribed method of reading is satisfactory. As the result of long ex-perience, we now use for greater accuracy several refinements of thismethod, as follows
:
(a) A special reading glass is used. It consists of a small eyepieceof about 4 cm. focal length, mounted so that it can easily be heldagainst and moved along the thermometer stem. In the focus exactlyin the center of the field is a sharp needle point. By taking readingswhen the needle point is opposite the top of the mercury column,parallax errors are eliminated.(b) Any simple device to beat regular intervals (such as i, 5, or loseconds) permits the observer to concentrate on reading the ther-mometer instead of trying to read both watch and thermometer at thesame time. Such a device also eliminates possible error due to eccen-tricity of the second hand of the watch.(c) The instrument is set out in the sun at least 15 minutes be-fore starting to read, and the shutter opened to the sun for about iminute during this period. In making a series of observations, thesecond set of readings is started 20 seconds after completing thefirst set of six readings. Thus a 4-minute shaded period occurs be-tween each 2 minutes of exposure. Each set of six readings is quiteindependent.II. For example:
NO. 14 ABBOT SILVER-DISK PYRHELIOMETER ALDRICII 716. (Note.) The approximate method of procedure stated in (13)and (15) is much easier than the exact method, and having beenfound by experiments to yield closely comparable results under allcircumstances of use, within the error of measurement, it has beenadopted, and the standardization of the instrument is made by thismethod.17. To reduce the result R^i to standard calories per square centi-meter per minute, or to the Smithsonian scale of 1913, multiply bythe factors furnished from the Smithsonian Institution with each in-strument.18. Example of reduction :Number i 2 3 4 5 6Reading I5?I2 I4?25 i4?8o I7?s8 I7?36 i6?09Differences 0^87 2?78o i?27Cooling correction.. i?070Ri .... 3-850Tx .... i6?2Scale correction . .
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—0.0020Ri= —o?oo8KR(T-30°) -(o.ooii)(i3.8)Rt= -o?058Air correction +o.ooo7Ri= +0?003RS .... 3?787PRECAUTIONSConstant watchfulness and care are needed to keep the instrumentin best condition. For example, it should not be inverted or jarred,thus keeping the mercury column and the wax bond betweenthermometer and silver disk intact. The blackening of the silver diskshould be examined periodically. If there is evidence of specks orspottedness, the surface should be carefully brushed with a softcamel's hair brush fastened to a long slender handle. The cover atthe end of the tube should be kept closed when not in use. Withcare the blackness remains unchanged over many years. Our twosubstandard silver-disk instruments, A.P.O. No. Stis and S.I. No. 5are evidence of this. Repeated comparisons against the absolute water-flow pyrheliometer over a period of 20 years have shown no evidenceof change.A silver-disk pyrheliometer ready for shipment, including stand-ardization and boxing, costs the Smithsonian Institution about $200.While it is not desired to manufacture them extensively, it has beenour practice to prepare and sell these pyrheliometers to individualsand institutions likely to use them for valuable and regular solar ob-servations. The special reading glass above mentioned can be fur-nished for $10 additional.
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IllAdopted constants of silver-disk pyrhcliometersSmithsonian pyrheliometry , scale of 1913Instru- Presentment constant Location and remarksS.I. 1 0.3733 U. S. Weather Bureau, 1910. Modernized, 1929.S.I. 2 0.3743 Observatorie Physique Central Nicolas, St. Peters-burg, Russia, 1910 (loan) ; National Observatory,Rio de Janeiro, Brazil, 1912.S.I. 3 0.3625 Conservatoire des Arts et Metiers, Paris, France, 1911.S.I. 4 0.3713 Royal University of Naples, Italy, 1911. Destroyed inWorld War II.S.I. 5 0.3715 U. S. Department of Agriculture, Physical Laboratory,1910. Modernized, 1931. On loan to AstrophysicalObservatory since 1926.S.I. 6 0.3666 Oficina Meteorologica, Buenos Aires, Argentina, 1911.S.I. 7 0.3638 Do.S.I. 8 0.3774 Central Observatory, Madrid, Spain, 1912.S.I. 9 0.3738 Imperial College of Science and Technology, London,England, 191 1.S.I. 10 0.3762 Koniglich Preussisches Meteorologisches Institut, Ber-lin, Germany, 191 1.S.I. II 0.3769 Meteorological Observatory, TenerifFe, Canary Is-lands, 1911.S.I. 12 0.3631 Koniglich Preussisches Meteorologisches Institut, Ber-lin, Germany, 1912.S.I. 13 0.3617 Schweizerische Meteorologische Centralanstalt, Zu-rich, Switzerland, 1912.S.I. 14 0.3721 University of Toronto, Toronto, Canada, 1912. Mod-ernized, 1937.S.I. 15 0.3609 National Bureau of Standards, Washington, D. C,1912.S.I. 16 0.3634 University of Arizona, Tucson, Ariz., 1912. Modern-ized, 1929. Sent to Service Botanique, Tunis, 1932.S.I. 17 0.3629 Harvard College Observatory Station, Arequipa, Peru,19 1 2. Sent to Smithsonian Observing Station,Calama, Chile, 1919. Modernized, 1929.S.I. 18 0.3774 Observatorio Nacional, Rio de Janeiro, Brazil, 1913.S.I. 19 0.3737 Aeronautisches Observatorium, Lindenberg, Germany,1913-S.I. 20 0.3657 Italian Indo-Asiatic Expedition, 1913. Returned toWashington, 1916. Repaired, 1919. Sent to Observ-tory, Helwan, Egypt, 1920.S.I. 21 0.371
1
Aeronautisches Observatorium, Lindenberg, Germany,1913-S.I. 22 0.3778 Observatorio Astronomico Nacional, Tacubaya, Mex-ico, 1913.S.I. 23 0.3683 Landwirtschaftliches Institut, Moscow, Russia, 1914.S.I. 24 0.3713 Meteorological Observatory, Teneriffe, Canary Islands,1914.S.I. 25 0.3717 Do.
NO. 14 ABBOT SILVER-DISK PYRHELIOMETER ALDRICH 9Instru- Presentment constant Location and remarksS.I. 26 0.3741 Meteorological and Geophysical Service, Batavia, Java,1 91 4. Modernized and returned, 1947.S.I. 27 0.3679 Manila Observatory, Manila, P. I., 1915.S.I. 28 0.3639 Meteorological Office, London, England, 1915.S.I. 29 0.3674 Calama, Chile, Smithsonian South American Expedi-tion, 1918.S.I. 30 0.3622 Do.S.I. 31 0.3748 Jewish Consumptives Relief Society, Edgevi^ater, Colo.,1920. Repaired, modernized, and sent to CarnegieInstitution, Colorado and California, 193 1.S.I. 32 0.3691 Mount Harqua Hala, Ariz., Smithsonian Solar Ob-serving Station, 1920.S.I. 33 0.3755 Reale Osservatorio Astronomico di Capodimonte,Naples, Italy, 1921.S.I. 34 0.3730 Riverside College Observatory, Sydney, Australia,1 92 1. Turned over to Commonwealth Observatory,Mount Stromlo, 1946.S.I. 35 0.3648 Do.S.I. 36 0.3640 Institutul Meteorologic Central, Bucharest, Roumania,1922.S.I. 37 0.3648 University of Lemberg and Warsaw, Lemberg, Po-land, 1922.S.I. 38 0.3726 Argentine Meteorological Service, La Quiaca, Argen-tina, 1924.S.I. 39 0.3764 Do.S.I. 40 0.3553 Institut de Physique du Globe, Paris, France, 1924.S.I. 41 0.3697 Observatory on Zugspitze, Germany, 1924 (loan).Modernized, 1927. Sent to Institute of Meteorology,Nanking, China, 1929.S.I. 42 0.3679 Mount Harqua Hala, Ariz., 1923. Modernized, 1932,and sent to Carnegie Institution, Colorado and Cali-fornia, as substitute for S.I. 16.S.I. 43 0.3735 Stellenbosch University, South Africa, 1924.S.I. 44 0.3793 Meteorological Service, Rio de Janeiro, Brazil, 1925.Modernized and returned to Brazil, 1928.S.I. 45 0.3658 University of Aberdeen, Scotland, 1925.S.I. 46 0.3705 Meteorological Bureau, Riga, Latvia, 1925.S.I. 47 0.3735 Smithsonian Solar Observing Station, Mount Bruk-karos. Southwest Africa, 1926. Smithsonian SolarObserving Station, Calama, Chile, 1932. Note.—No. 47 and No. 48 have special 8o-cm. tubes.S.I. 48 0.3827 Do.S.I. 49 0.3586 Battle Creek College, Mich., 1927.S.I. 50 0.3800 Physikalisch-Meteorologisches Observatorium, Davos,Switzerland, 1927. Repaired and returned to Davos,1934-S.I. 51 0.3755 Meteorological Office, Simla, India, 1927.S.I. 52 0.3787 University Observatory, Kiel, Germany, 1927.
10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill
Instru- Presentment constant Location and remarksS.I. 53 0.3797 Commonwealth Solar Observatory, Canberra, Aus-tralia, 1928. Repaired and returned to Australia,1937-S.I. 54 0.3824 Commonwealth Solar Observatory, Canberra, Aus-tralia, 1928. Repaired and returned to Australia,1939-S.I. 55 0.3848 University of Illinois, Department of Botany, Urbana,1928.S.I. 56 0.3742 Universitats-Sternwarte, Kiel, Germany, 1928.S.I. 57 0.3818 Institut Scientifique Cherifien, Rabat, Morocco, 1929.S.I. 58 0.3831 Oporto Observatory, Portugal, 1929.S.I. 59 0.3851 University of Arizona, Tucson, 1930.S.I. 60 0.3945 American Society of Heating & Ventilating Engineers,Pittsburgh, Pa., 1931. Repaired and returned toPittsburgh, 1945.S.I. 61 0.3867 Meteorological Service of Martinique, Fort de France,1932.S.I. 62 0.3960 Do.S.I. 63 0.3821 Blue Hill Meteorological Observatory, Milton, Mass.,1932. Repaired and returned to Blue Hill, 1941.S.I. 64 0.3870 Laboratoire Actinometrique de I'Observatorie deTrappes, France, 1933.S.I. 65 0.3913 Institute of Meteorology, Nanking, China, 1933.S.I. 66 0.3868 Institut de Physique du Globe, Paris, France, 1934.S.I. 67 0.3877 Tananarive Observatory, Madagascar, 1935.S.I. 68 0.3788 Institut de Physique du Globe, Paris, France, 1935.S.I. 69 0.3849 Geophysical Observatory, Pilar, Argentina, 1936.S.I. 70 0.3816 Do.S.I. 71 0.3737 Commonwealth Solar Observatory, Canberra, Aus-tralia, 1936. Repaired and returned to Australia,1937.S.I. 72 0.3820 Commonwealth Solar Observatory, Canberra, Aus-tralia, 1936. Repaired and returned to Australia,1939-S.I. 73 0.3789 Eppley Laboratories, Newport, R. I., 1937. Newthermometer inserted and returned, 1939.S.I. 74 0.3962 University of Minnesota, Minneapolis, Minn., 1937.S.I. 75 0.3847 R. Dvorak, Agent, Prague, Czechoslovakia, 1938.S.I. 76 0.3876 Observatorio Central Meteorologico, Lisbon, Portugal,1940.S.I. T] 0.3922 Geophysical Institute, University of Coimbra, Portu-gal, 1940.S.I. 78 0.3926 U. S. Weather Bureau, Washington, D. C, 1944.S.I. 79 o.2,7Z^ Physics Institute, Helsinki University, Finland, 1946.S.I. 80 0.3776 Hebrew Technical College, Haifa, Palestine, 1947.S.I. 81 0.3871 Institute for Advanced Learning, Dublin, Ireland, 1949.
NO. 14 ABBOT SILVER-DISK PYRHELIOMETER ALDRICH IIVarious A.P.O. instrumentsInstrument Constant Location and remarksA.P.O. IV 0.5118 Copper disk. 1906. Mount Wilson, Calif.A.P.O. VII 0.5072 Copper disk. 1906. Alount Wilson, Calif.A.P.O. VIII 0.5150 Copper disk. 1906. U. S. Weather Bureau andMount Wilson.A.P.O. 8 0.3760 First silver-disk instrument. 1909. Washing-ton and Mount Wilson. Repaired 1910 andcalled A.P.O. 8b. s.A.P.O. 8bi 0.3786 Modernized 1927. Reserved at Washington forcomparisons only.A.P.O. 9 0.3631 Built 1910. Used at Washington, Mount Wilson,Mount Whitney, Algeria, Arizona. Modern-ized 1928 and called A.P.O. 9bi8.A.P.O. 9bi» 0.3684 Zentralanstalt fiir Meteorologie und Geodyna-mik, Vienna, Austria, 1928.A.P.O. 10 0.3720 Built 1923. Sent to Harqua Hala and TableMountain.A.P.O. II 0.367s Sent to Mount Brukkaros, Southwest Africa,1925. Modernized and sent to Mount St.Katherine, Egypt, 1933. Repaired 1938 andsent to Tyrone, N. Mex., Miami, Fla., andTable Mountain, Calif.A.P.O. 12 0.3618 Table Mountain, Calif., 1928.A.P.O. 13 0.3893 Mount St. Katherine, Egypt, 1932. Rebuilt 1938and sent to Tyrone, N. Mex., Miami, Fla., andTable Mountain, Calif.A.P.O. 14 0.3854 Mount St. Katherine, 1932; Tyrone, N. Mex.,Miami, Fla., Table Mountain, Calif.

SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill, NO. 14, PL.
Abbot Silver-disk Pyrheliometer, with Long tube adopted i927