Subject: Re: IN DESPARATE NEED OF HELP (Photophone)

From: STEINMETZ@ (Jim Worthey)
Date: 3 Nov 1995 14:47:22 GMT

In <[email-masked]>, Zbigniew Wolanski <[email-masked]> writes:
>I am having trouble with my science fair project, it is based on the photophone.
>I would like it very much if someone could give me a describtion of how it works, 
>who invented it, etc.
>					IN NEED,
>					    Filip Wolanski

Filip

I believe that one photophone design was developed by Alexander
Graham Bell. There was a practical article, or a series of articles,
on photophones in Popular Electronics, or some practical magazine
like that. I'm pretty sure the author was Forrest Mims. That is, it
was probably part of a regular column that Forrest Mims did (or
does). If you do a computer search on that name, you'll find that 
he writes lots of stuff, and that in the late 80's he did a couple
articles about photophones.

Jim Worthey

---

From: [email-masked] (Ian Ashdown)
Date: Fri, 03 Nov 1995 04:47:32 GMT

>Jim Worthey

If by "photophone" you mean a telephone with a light beam modulator
and receiver in the communications path, then take a look at:

  Baldwin, Neil. 1995. "The Laboratory Notebooks of Thomas Edison,"
  Scientific American 273(4):160-163 (October)

On page 160B, the article reproduces a page from Edison's notebooks,
dated April 5, 1881, that reads: "Telephony by light by use of
tasimeter." The caption reads:

  As this bizarre drawing (for a construction that never became a
  reality) demonstrates, Edison was equally capable of disregarding
  evolution from one invention to the next. The tasimeter was a
  thermometer-based instrument that Edison created to quantify the
  heat given off by the sun's corona during a total eclipse on July
  29, 1878. The most important component of the tasimeter was a
  compressed carbon "button" that expanded and contracted in
  response to minute variants [sic] in temperature. Edison employed
  this same carbon button in his telephonic transmitter (an
  improvement on Alexander Graham Bell's invention) because of
  its comparable sensitivity to the pressure of spoken-voice
  sound waves. He fancifully imagined "marrying" the two
  inventions, although he had no idea how to transmit telephone
  signals by light.

Of course, a *patentable* invention has to be reducible to practice,
so Edison is not the true "inventor" of this device. However, this
does appear to be the first documented discussion of the concept.