30 Apr
2011
30 Apr
'11
1:39 a.m.
On 4/27/2011 2:33 PM, Tony Duell wrote:
OK, thanks.
Usually the search engines guess better than on this one.
After all, only one "r" was missing! Neither Ixquick nor Google guessed
the other spelling though.
On 4/26/2011 2:17 PM, Tony Duell wrote:
<snip>
I'm sure it's just a typo, and I apologize in advance for being too dumb
to figure it out, but what is a baretter? (Sounds quite interesting,
It is a typo, or rather a mis-spelling. It should be 'barretter;. The
reason is quite simple--tungsten-filament lamps (not that I
expect anyone to have any carbon-filament ones) have a very steep
resistance-temperature curve (higher "hot" resistance).
Tugsten filament lamps approximate a constant current load over quite a
range of voltages (am I the only person to rememebr the baretter?).
A barretter is a constant-current device, essentially a special type of
filament lime. Often the filament was iron wire in a hydrogen-filled bulb
(or so I've read). The current floowing though the device was fairly
constant for quite a wide range of voltages across it.
Some UK series-string radio sets had one in series with the heater chain
(rather than a dropping resistor) so that the heater current would be
correct for jsut about any mains input voltage.
The were also used as a ballast for the Nernst glower in some IR sources
(which is where I first came across one at school [I managed to convicne
the powers that be that getting me to chase a ball around an area of
grass was a waste of everyone's time, so I got to fiddle with electronics
stuff instead. Getting the old IR source to work again was just one of
the many things I tried]
Wow, I'm sure you are correct in some incarnations of the device, but my
quick search shows Fessenden using it as what sounds like an AM
detector. There is also mention of using it for microwave power
measurements. I suspect these are very delicate versions of the ones
used as constant current devices with probably a different method of
construction. One I found (Wikipedia - yes I know that is suspect)
starts with a .003 inch diameter platinum wire encased inside a 0.1 inch
diameter silver wire. Then the combination is drawn down until the
outer silver wire is only .002 inch in diameter. By this point, the
platinum is very thin indeed. Next a bit of the silver is etched away,
leaving a very thin section of platinum as the operational element. The
thing is then put into an evacuated glass bulb with the silver leads
brought out. Supposedly the tiny platinum section can change resistance
quickly enough to follow the audio modulation but not the RF carrier on
which it is superimposed. Probably not used too much in classic
computers, but fascinating nevertheless.
FWIW, if you have an old device using a barretter, you
can often use a
mains-voltage lamp to replace it. I am pretty sure I used a normal 60W
bulb (so 0.25A at 240V) to replace a 0.3A barrretter in said IR source.
-tony