WESTERN IJNIVERSITY OF PENNSYLVANIA.

ALLEGHENY, PA., APRIL 24th, 1895

PROFESSOR EDWIN J. HOUSTON, CHAIRMAN COM-MITTEE ON INCOMPLETED CONGRESS WORK,PHILADELPHIA, PA.

DEAR SIR: In reply to your request for a statement of the results obtainedby me in regard to the work on standards of light and illumination, I would makethe following report.

1. Secondary Standard.: -The result of my work on incandescent lampshas shown that a set of two incandescent lamps, mounted so as to invariablyhave the same point turned toward the photometer spot, as faced it when thelamnp was originally stan-dardized, and worked at tlhe same voltage as that atwhich the standardizing was done, can be used with very considerable accu-racy for secondary standards. As mentioned in a previous report, one lamp isused for the photometric work, and the other is used as a check, and when thereference lamp has been burned a total of 200 hours, the set is to be discarded.The curve of fall of candle-power with life can be used for making corrections.

2. Primary Standards:-Work on the acetylene standard mentioned in aprevious communicationi has been prosecuted under difficulties, as I had to Inakemy experiments with barium carbide, and could, up to a week ago, obtain butsmall quantities of the gas. 1 have since been able to obtain a sufficient quan-tity, and have made numerous experiments.So far, my attention has been given to the best means of keeping the gas at a

uniform pressure, and on the best pressure to use. I have devised a convenientand satisfactory method of accomplishing the former, annd enclose a drawing ofthe apparatus at present in use. As regards the latter, I find that a tolerablyhigh pressure gives better results as regards constancy, than a low one, the pres-sure at present used being 12 inches of water.By reason of the results obtained by Captaini Abney, who showed that a flat

flame gives a rnuch more eveni illumination than a round one, I have been usinga flat flame. The experiments so far made show a very constant illumination,and this without using a chimnney.The experiments about to be made will be on the following points-a. effect

of impurities in the gas; (b) best means of obtaining flat flame; (e) possibilityof accurately reproducing standard absolutely, by using slot of definite size anddefinite mode of generating and using.

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FESSENDEN'S REPORT' 501

I may say that an appropriation of $800.00 has been made by the universityfor the purpose of making these experiments.

In conclusion, I would state that the acetylene standard appears to me tohave the following advantages, provided that further experiments show it canbe duplicated successfully.

1. Use of definiite chemical compound, on the line suggested by Mr. Ken-nelly at the meeting of December, 1893. Even if different specimens of calciumcarbide should be of different degrees of impurity, there is no reason to doubtbut that these impurities can be absorbed, and pure acetylene left.

2. It is self-contained, the only materials used being calcium carbide andwater, and no connection to gas pipes are necessary.

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A V l

FIG. 1. -Fessenden's Report.A. Generating Apparatus. W. Wash Bottle. G. Guide Rod.C. Carbide of Calcium. P. Gasometer. F. Flame.S. Strainer. V. Regulating Valve. H. Slot.

3. As the substance burned is of a definite chemical composition, and the di-mensions of the slot and all the apparatus is capable of being accurately definedin terms of length and pressure, it seems to afford a method of obtaining anabsolute standard.

4. The light appears to be much whiter than ordinary gas, which is a point ofconsiderable irnportance in a standard. No exact spectro-photometric measure-ments have as yet been made, however.

I remain, very truly yours,REGINALD A. FESSENDEN.

[Presented to Council May 21st, 1895, and ordered to be printed in theTRANSACTIONS.]