On Tue, 22 May 2012, David Jonsson wrote:
Thanks. I will try to build one.
Will this transistor do?
http://www.newark.com/nte-electronics/nte451/transistor-jfet-n-channel-4ma-i/dp/29C4598
Probably, but you'd have to try it. Or just use MPF-102:
http://www.newark.com/fairchild-semiconductor/mpf102/rf-jfet-n-channel-15v-to-92/dp/21K5272
Mouser has them, also Radio Shack.
An idea is to build an array of these and measure with a cheap
microcontroller.
That FET circuit is a detector only, not a linear meter with gain and
zero. Ridiculously Simple Charge Detector is extremely crude/cheap so kids
can build one themselves. Also, like all passive electrometers it only
detects AC, although more complicated ones can go down to tens of seconds
time-constant or longer.
If you can afford more than $3, or know some electronics, then look at
the other examples in LINKS http://amasci.com/emotor/chargdet.html#15
For actual DC field detection, "field mill" electrometers are required.
They use a small grounded spinning blade placed near the electrometer
antenna to chop any constant e-field into AC. That way the empty space
surrounding the antenna doesn't behave as a series-capacitor which blocks
all long-term DC signals.
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William J. Beaty SCIENCE HOBBYIST website
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