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    "label": "Letters Sent to Mr. Markley [E.A. Markley], no date",
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    "id": "75730655",
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    "title": "Letters Sent to Mr. Markley [E.A. Markley], no date",
    "citationUrl": "https://catalog.archives.gov/id/75730655",
    "collections": [
        "Safety Light Collection",
        "Records Related to Radium Dial Painters"
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    "seq": 2,
    "pageIndex": 0,
    "type": "document",
    "url": "https://s3.amazonaws.com/NARAprodstorage/lz/electronic-records/SLC/Radium/SLC_0003676.pdf",
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    "ocrText": "Dr. Paul D. Berrisford,\n810 Lowry Bldg.,\nSt. Paul, Minn.\nDear Dr. Berrisford:\nDr. Lind has written me relative to your interest in\nthe use of radium luminous discs as a weak light source.\nDr. Lind has ably advised you relative to the physical\ncharacteristics of our luminous compound. The light intensity of this\nmaterial decreases with time, and while it is true that the higher grades\nof our Undark self luminous material has a more rapid decrease than the\nlower grades, still I doubt if our lower grades would present a uniform\nenough source of light to be of much value in your work. Even the\nmaterial which has stood for a considerable period of time and had an\nopportunity to age, so to speak, would still have an appreciable decay curve.\nI infer from Dr. Lind's letter that you are interested\nin a fairly constant weak light source, and if such is the case there is\nconsiderable question in my mind regarding the feasibility of using radium\nluminous compound. I an, however, having our Mechanical Department paint\nup a few discs which we will forward to you during the next few days under\nseparate cover, for your experimentation.\nThe intensity of light from the lower grade disc will\nprobably remain quite constant over a short period of time, but after\nseveral months will show some change.\nIn connection with problem presented, I am wondering if\nthe best solution would not be to utilize a very weak light source such as\nmay be obtained with the proper set up of resistance etc. In this connection\nwe have designed a pioce of equipment for reading our luminous material which\nmight very well be adapted to your use. The intensity of the light from\nthis apparatus is of the order of magnitude, naturally, of the luminous compound.\nThe light intensity may be veried anywhere from one to 250 microlamberts.\nThis apparatus we have calibrated against a known light source and when in use\nit would be necessary to check this calibration oceasionaly on account of\ndeterioration of the fildments in the lamp. This would not be difficult,\nhowever as the apparatus could be supplied with accessory equipment so that it\nwould be possible to have the calibration checked by a suitable laboratory with-\nout énterferring with the daily use of the equipment.\nIf you should be interested in this type of equipment I\nwould be glad to go into the matter in more detail with you.\nVery truly yours,\nHHBarker:RH\nVice President."
}