[D8933AAM1], Letter from Harold P Brown to Thomas Alva Edison, February 14th, 1889


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[D8933AAM1], Letter from Harold P Brown to Thomas Alva Edison, February 14th, 1889

Editor's Notes

My dear Mr. Edison, I have frequent occasion to test arc lights in behalf of city authorities who have contracted for them as lights of 2,000 candle power, and these are not always satisfied with emasurements of the volts an d amperes of the current. As I always leave the mayor and council pledged to a ordinance prohibiting the alternating current at higher E.M.F than 500 olts, I trust you will pardon my addressing you concerning the practicability of a photometer for getting merely approximate measurements from the lamps as they appear in service. The only feature of novelty is that a pane of colored glass is set at an angle midway between the angles of light rays received from both sources; parallel with this is a sheet of ground glass, one half of which is illuminated from each source. As the lights are then of the same color, the distance of the standard light can be varied unfit the line of demarcation between the halves is lost. By setting the bottom "ed" level and arranging the apertures so that angle "bfa" must be of say 40 degrees (for "long arc" lamps) in order that the light from a shall strike the screen "f", the length of "af" may of course be determined by measuring the base of the triangle, not know whetehr the color in the glass "e" will have a arying effect upon the light from the different sources and, if so, what the variation would be. It will add greatly to my obligation s to you if you can see that I am set right in the matter, and will indicate some candle or portable lamp to be used as a standard.####Sincerely Yours, Harold P. Brown.####[[next page: There follows a diagram of a plan for a photometer]




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Thomas A. Edison Papers, School of Arts and Sciences, Rutgers University
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