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What is the iodine fax process mentioned in Vannevar Bush's "As we may think"?

Another process now in use is also slow, and more or less clumsy. For fifty years impregnated papers have been used which turn dark at every point where an electrical contact touches them, by reason of the chemical change thus produced in an iodine compound included in the paper. They have been used to make records, for a pointer moving across them can leave a trail behind. If the electrical potential on the pointer is varied as it moves, the line becomes light or dark in accordance with the potential.

This scheme is now used in facsimile transmission. The pointer draws a set of closely spaced lines across the paper one after another. As it moves, its potential is varied in accordance with a varying current received over wires from a distant station, where these variations are produced by a photocell which is similarly scanning a picture. At every instant the darkness of the line being drawn is made equal to the darkness of the point on the picture being observed by the photocell. Thus, when the whole picture has been covered, a replica appears at the receiving end.

I searched for "Vannevar Bush iodine facsimile" and many other keyword combinations. I also asked GPT. I found nothing.

My guess would be that it's a redox reaction like $2 KI \to I_2 + 2 e^- + 2K^+$.

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  • $\begingroup$ Not very clear... Facsimile transmission or telecopying used Thermal paper: I'm not sure if the author is referring to some specific type of paper (patent or similar) or if he is simply considering this example in general. $\endgroup$ Commented Jun 15, 2023 at 8:50
  • $\begingroup$ This IMO is the reason why the query "Vannevar Bush iodine facsimile" gives no answer: there is no reason to think that Bush was involved in some way in developing fax technology. $\endgroup$ Commented Jun 15, 2023 at 8:51
  • $\begingroup$ historictech.com/… says a potassium iodide and starch solution. $\endgroup$
    – Jon Custer
    Commented Jun 15, 2023 at 17:36

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I suspect that Bush meant things like the Fultograph (as one example). They used paper that had been soaked in potassium iodide and starch. When electricity is passed through the paper, the iodine is released from the potassium iodide and binds to the starch, making a blue/purple color. So-called potassium iodide starch strips are used to test for oxidizers using the same reaction (but chemically driven, not electrically).

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