June, 1903. Monthly Weathex Review
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JUNE,1903. MONTHLY WEATHEX REVIEW. 276 While some of these displacements must be attributed to resistance (which could be made Q) ) and M the ammeter sup- temperature changes and effects entirely within the instru- plied with the apparatus. R is a resistance box of considera- ment, yet slow tiltings of the ground also occur, due to a ble size, which was used, partly to cut down the current on variety of causes. The seismograph, as now installed, answers cloudy days, and partly to keep two Daniels’ cells (B) in con- every purpose for the registration of distinctively earthquake dition when the apparatus was not in use. T is a mercury movements, but the slow tiltings referred to can not be three-way key. studied satisfactorily in the present location of the apparatus *Earlyin December the behavior of the observing tube be- which for such purposes should be isolated as far as practicable. came very irregular, its resistance often becoming infinite for no apparent reason whatever. It was, therefore, returnect to OBSERVATIONS OF SOLAR RADIATION WITH THE Washington and its contacts thoroughly examined, and, al- AN(3STROM PYR-LIOMETER, AT PROVIDENCE, R. 1.l though no trouble could be found, the bad contact was in By Mr. HARVEYN. Davrs, dated Marrh 9.1903. some way improved, for it functioned properly during the rest of the year. During the fall of 1901 arrangements were macle by Prof- During the summer of 1902 the writer was obliged to give Cleveland Abbe, on behalf of the United States Weather up the work on account of his absence from the country, and Bureau, and Prof. Carl Barus, of Brown University, for mak- Mr. Robinson Pierce, jr., also of Brown University, very kindly ing a series of observations upon the amount of solar radiation undertook it. The apparatus was moved to a similar situation received from day to day at the surface of the earth. AJI at his home. a quarter of a mile away on the same ridge, the hgstrom electric compensation pyrheliometer, No. 28, and a altitude of the tube being 163 feet. Here observations were Weston milliamperemeter, No. 4315, were accordingly sent to macle during July, August, and a part of September. Others Providence, E. I., and the work placed in my hands. were made later in September at the first place. Besides As an observing station we finally decided upon a room in making these observations, Mr. Pierce has also carried through the third story of a house situated in one of the highest parts a considerable number of the calculations, whose results appear of the city. The galvanometer, resistances, and batteries were in the accompanying tables. permanently fastened to the wall just inside a southern wiiidow, The method of observing was as follows: The tube was first while the sloping roof outside offered a convenient and ex- set up and oriented, both strips being esposed to the sun, and posed support for the tripod and pyrheliometer. When in the standard time, the neutral reading of the galvanometer position the observing tube was about 18s feet above sea level. (the key being open) and the temperature inside the tube were As is already well known,’ the instrument consists essentially recorded. The “throw ” when the key was closed was also of two thin narrow strips of blackened platinum so mounted observed, and both strips were exposed until this was a mini- as to be exposable to the sun’s radiation. While one is thus ~nu~n,and usually very small. Two current determinations exposed the other is shielded and heated to the same tempera- mere then made, the first with the left-hand strip in the cir- ture by the passage of an electric current of known intensity cuit ani1 the second cL switch right; ” the tiibe’s orientation (usually .2 to .4 amperes), the ammeter and a variable resist- was corrected; two more cletermiiiations were made, the first ance being included in the circuit. The desired equality of ‘’ switch right ” and the second “ switch left,” ancl then the temperature is recognized by means of a secondary thermo- time, teniperature, and zero point were again observed and electric circuit, including a very sensitive galvanometer of the rec*orcled. The mean of four such current determinations was D’Arsonval type, and a constanten-copper thermal element, taken as the i of the set, corresponding to the mean tinie and whose junctions are very close to, but electrically insulated from, the mean temperature. The total time necessary to complete the centers of the two strips. At first the instrument was used a set was from four to eight minutes. The state of the sky with its electrical connections just as they were packed, but a was also recorded. considerable shifting of the zero point of the galvanometer The sources of error to which such work is subject are very soon appeared, and seemed to be due to a bet in the torsion many. In the fir& place, a brisk breeze, if it were from the suspension, caused by the estreiue deflections to which SO sen- right direction ancl a bit gusty, was sometimes enough to sitive an instrument is liable, before the current strength can cause a throw of 2 or 3 centimeters in a male distance of be ~roperlyregulated. On this account I was led to introduce some 50 centimeters, and the resulting error in the determi- a platinum key into the galvanometer circuit and to use a zero nation of radiation is 5 or 10 per cent. It is almost always method, adjusting the current in the main possible, however, to take readings between times when the wind circuit until no throw was observable when is gusty; when it is steady, the effect upon the mean 1 should the key was closed. This key was almost be zero, so that this trouble is not particularly formidable if immediately replaced by a mercury com- one does not care for accuracy within say 2 or 3 per cent. A mutator, symmetrical with respect to the further difficulty is caused, on all but the best days, by varia- galvanometer ancl iiyrheliometer tube, to tions in the amount of heat absorbed by mists or clouds in avoid any spurious thermal E. &I. F. in the the path of the Run’s rays. If the cloud layers are at all circuit. It was also found convenient to thick, the resulting fluctuations in the radiation received are modify the connections of the main circuit so considerable aid so rapid that anything but the roughest for various reasons, until it assumed a form kind of an approximation is at once impossible and meaning- schematically represented in fig. 1. P is Frit. 1. less. The presence of either of these difficulties is indicated the tube, C: the commutator, and (3 the galvanometer of the in the nccompanying tables by the words “ readings variable,” thermo-couple circuit. In the main circuit r is the variable and when a full set of four determinations could not be ob- tained the resulting radiation number is marked with ( ? ). 1 A similar report by Mr. H. H. Xiriball will follom.-ED. 2 See Prof. C. F. Marvin: ‘* The measurement of sunshine and the pre- INTERTICtATION OF INSTRUMENTAL ERRORS. liminary examination of Angqtriirn’s pyrheliometer, ‘’ MONTHLYWEATHER REVIEW, October, 1901. Besides these meteorological troubles there were also in- See also Knut .hg.strijm, Intensit6 de la Radiation Aolaire-Reclierches strumental ones to be reckoned with. The most obvious of faites A T6n6riffe, 1895 et 1836. Uysal, 1900. these was a scale error in the ammeter, the pointer of which See also K. Angstriim, Nova Acta Upsal, 1893: The Physical Review, quite evidently read some 0.008 amperes too low when the in- 1, p. 365, 1893: Wied. Ann. 67, p. 636. 1899; Astrophysical Journal. 9, p. 334, 1899, and Annalen der Physik uncl Chemie, Neue Folge, Baud 67 [ 18991, strument was first received. It was accordingly connected in p. 633. series with a Thomson current balance, No. 131, a variable re- 37-3 Unauthenticated | Downloaded 09/28/21 12:58 AM UTC 276 MONTHLP WEATEl3R REVIEW. JUNE,1903 sistance (that supplied with the apparatus), and from one to in general, considerably higher than the corresponding figures fifteen cells of a large storage battery in the Brown laboratory, for strip R, under the same conditions. All the sets which and several sets of comparisons were made covering the whole were obtained in 1902, under favorable meteorological condi- scale. In all, except the first few, the pointer of the ammeter tions, have been examined with this in view. If we call the was brought by means of the resistance esactly over a chosen four readings of a set, in their order A, B, C', and U (A and D scale division, which could be done with great accuracy. and being- taken with the current through- L),,. then the Quantitv the current balance was then adjusted and read to the nearest A-fD B+C L-R=--------- was found to have an average value tenth of the smallest division of its accurate scale, the results 2 a being reduced by means of the tables supplied with the instru- of + 22.6 thousandths of an ampere, and it was negative in ment and being accurate to within considerably less than one only five sets in a total of 232. The cause of this is still ob- thousandth of an ampere, beyond which it was impossible to scure. Of course such a discrepancy woulcl arise if the strips read the ammeter in actual use.