Hi, the maltese cross is a chopper to interrupt the lines of electrostatic force. The cross could have a hundred legs, as long as it alternately blocks and unblocks exposure to the electric field. The cross spins in a horizontal plane, maybe half an inch above the sensor electrode which is flush or just above the ground plane. The idea is to get the output signal up to a frequency where the time constant of the sensing electrode can be quite short. If you can get it up to 400 Hz it only needs a 2.5mS time constant. Even then, since you can calibrate it you can have the signal below the time constant. A 12 legged "cross" spinning on a 4 pole motor gives a chopping frequency of 360 hertz, (in America), quite convenient for amplification and processing. If the motor is synchronous, or you put a sensor on the motor shaft you can run a phase sensitive detector and get polarity as well as magnitude. cheers, Neville Michie
On 27/06/2014, at 1:59 AM, Max Robinson wrote: > How fast does the maltese cross turn? > > Regards. > > Max. K 4 O DS. > > Email: m...@maxsmusicplace.com > > Transistor site http://www.funwithtransistors.net > Vacuum tube site: http://www.funwithtubes.net > Woodworking site > http://www.angelfire.com/electronic/funwithtubes/Woodworking/wwindex.html > Music site: http://www.maxsmusicplace.com > > To subscribe to the fun with transistors group send an email to. > funwithtransistors-subscr...@yahoogroups.com > > To subscribe to the fun with tubes group send an email to, > funwithtubes-subscr...@yahoogroups.com > > To subscribe to the fun with wood group send a blank email to > funwithwood-subscr...@yahoogroups.com > > ----- Original Message ----- From: "Neville Michie" <namic...@gmail.com> > To: "Discussion of precise time and frequency measurement" > <time-nuts@febo.com> > Sent: Wednesday, June 25, 2014 10:46 PM > Subject: Re: [time-nuts] Loran, GPS, Lightning, Timing > > >> Is anyone using a field mill? >> I have always been going to make one. >> It consists of a horizontal metal plane with a conducting button on the >> surface >> which is insulated from the ground plane. >> A metallic maltese cross driven by a motor alternately covers and uncovers >> the electrode >> exposing/not exposing it to the sky. >> The electrode button has capacitance to ground which drops its impedance, >> but across that impedance is an AC voltage proportional to the ambient >> static field. >> A typical field in fine weather is 300 Volts/metre so the signal is not >> trivial. >> You should be able to make a field mill that works continuously except when >> actually shorted by rain. >> This is not a very high impedance device, and should show many marvellous >> things >> as the clouds float over you. >> You can calibrate it with a metal plate, say 12 inches above it with 50 >> volts on it. >> That should produce a uniform field on the mill. >> cheers, >> Neville Michie >> >> >>> >>> I know you are talking about measuring lightning strikes but if you get the >>> impedance high enough, you can actually measure the earth's electric field. >>> (It is about 200V/m if I recall properly.) Interestingly it is affected by >>> the solar flux and solar wind. >>> >>> -- >>> Brian Lloyd >>> Lloyd Aviation >>> 706 Flightline Drive >>> Spring Branch, TX 78070 >>> br...@lloyd.com >>> +1.916.877.5067 >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
