Date: Fri, 23 Aug 2002 15:21:06 -0400 (EDT)
From: "Keith F. Lynch" <kfl at KeithLynch.net>
To: WSFAList at KeithLynch.net
Subject: [WSFA] Re: Looking backward
Reply-To: WSFA members <WSFAlist at keithlynch.net>

> But I wonder how amplification could have been accomplished in 1900,
> absent vacuum tubes.

You can connect a dynamic microphone directly to a speaker.  For
instance you can connect two telephone earpieces with a length of
wire.  Someone who shouts into one can be faintly heard in the other.
But as you point out, there's no amplification.  No more sound
energy can come out of the speaker than went into the microphone.
If you have many speakers playing the same sound, this is obviously
unworkable.  (Except maybe for bagpipe music.  Bagpipes are LOUD.)

However, the very first practical telephone involved amplification.
Bell's original microphone included a needle dipped into sulfuric
acid.  A sort of variable battery.

This had some problems, however.  The reason he called out to Watson
for assistance is that he had spilled some of the acid on himself.
Even if he hadn't, the needle would have needed frequent replacement
as it kept dissolving in the acid.  Which would also need frequent
replacement.  I'm sure it was also quite noisy.

By 1880, the carbon microphone had been invented.  A variable
resistor.  Place it in series with a (sealed) battery and a speaker,
and the sound that comes out of the speaker has more energy than the
sound that goes into the microphone.  You can easily get a feedback
howl with just a telephone earpiece, carbon microphone, and dry cell
battery.  No transistors, no tubes.

Bellamy's cable radio would have been perfectly workable if it had
included several cascades of one speaker playing into several carbon
microphones, each with its own battery or other source of filtered DC.

Of course the audio quality would have left something to be desired.
There's a reason why carbon microphones are never used in high fidelity
work.  I'm not sure if they're even used in telephones anymore.
--
Keith F. Lynch - kfl at keithlynch.net - http://keithlynch.net/
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