Re: OT: the rubbing effect
- From: Johnny B Good <jcs.computersbutt@xxxxxxxxxxxxxxxx>
- Date: Wed, 27 Feb 2008 23:27:14 GMT
The message <aa06s3hm0v92iikhbrh0e7imd5buh9v3hj@xxxxxxx>
from Colin Stamp <col.dustbin@xxxxxxxxxxxxxx> contains these words:
I first read about this effect at primary school in a book about
Thomas Edison. Apparently, he'd used it to produce mechanical
amplification in a speaker for telephones.
I think there was a motor-driven drum, coated in some kind of slimy
liquid (Chalk-dust was mentioned, as I recall). A metal "finger"
rested against the drum and drove the centre of the diaphragm. The
signal was connected between the drum and the finger. It was very
loud, but the quality struggled to achieve DAB levels...
I reckon finger grease has the same properties as Edison's slime.
What you've just described sounds suspiciously like a _mechanical_
amplifier as used by some of the early record players pre-dating the
development of the thermionic vacuum tube (valve) amplifier.
This basically comprised a circular felt mat sandwiched by two discs.
The 'power supply' disk would be driven mechanically (either clockwork
or electric motor) at a regulated speed. The other (output) disk would
be connected to the diaphragm of a horn loaded pressure transducer and
be pressed against the felt matt by a bias spring so that the drag due
to this 'slipping clutch' would put pressure onto the diaphragm.
The vibrations picked up by the stylus were linked somehow to the
pressure plate so as to modulate the spring bias pressure, thus causing
a copy of the vibrations to be transferred at a much higher level into
the diaphragm of the horn loaded speaker, using the power provided by
the motor.
Although this worked well enough, it would have had a limited frequency
response and a high noise level (hiss) both such limitations being
matched by the input source so not a serious detriment in the
circumstances.
However, such a system could have used wet corn starch in place of the
felt pad and an electric bias field applied between the (in this case,
metal) disks with an electrical signal applied (either transformer or
capacitor coupling being used) to modulate the bias voltage.
Another alternative was to use the amplifying features of the humble
carbon microphone whereby the reciever diaphragm doubled up as the
carbon microphone diaphragm.
All three cases involved mechanical vibration as a necessary
intermediary step in the overall process of amplification.
I think that the two essential ingredients in the phenomena of the
"100Hz buzz" effect are a highish voltage (120 to 240v) and a very high
impedance which together create a 'constant current' source.
The effect requires a mechanical power source which, in this case, is
provided by the movement of the hand across the 'live' surface thus
mimicking a mechanically powered amplifier using electrical 'signal
input'. In this case, there is no bias voltage hence the 100Hz effect.
--
Regards, John.
Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.
.
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