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Charge Leakage and measuring Voltage with an Electroscope

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Antonio

Tue Oct 19 2010, 10:10PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The "FET electrometer" in the book above can be used to detect polarity or to sense the presence of a strong electric field, but is useless for quantitative measurements because the charge in the gate leaks too fast. The foil electroscope as shown is a common error in modern texts, and will not work well in this way. The foil just bent and suspended is too stiff for reasonable sensitivity, and the electroscope mounted inside an insulating plastic pot will charge the interior of the pot quickly, and the foils will not close if the electroscope is discharged. In a correct assembly the bottom of the pot must be conductive, and the side walls shall have strips of metal to drain stray charges. Here is one that works well:

ThereIsNoSky

Wed Oct 20 2010, 12:08AM

Registered Member #2423 Joined: Tue Oct 06 2009, 02:49AM
Location: Oklahoma City
Posts: 10

Patrick wrote ...

ThereIsNoSky wrote ...

@Patrick - I would prefer to make direct measurements on the circuit. My current plan is to put an inductor or resistor in series between the cap and ground. This will give me a very good low resistance Earth ground for my HV DC coupling on the electrostatic field. Then I would like to measure the voltage across the inductor or resistor to see any E-Field interactions. Do you think this will work? As my E-field is affected by other E-fields this should induce a small(?) voltage rise or dip across the capacitor. This change will then be measured by a scope on the inductor / (small value) resistor without draining any of the charge off of the HV side of the capacitor.

oh, man you've asked a very complicated question. i think you will have problems using a 1M/or 10M ohms scope impedenace. the resistor or inductor you describe should really be considered a antenna and its physical shape may matter a whole lot for intercepting any givin signal/field. and close, low field changes will be detected as the same as a far, High field. at least for a detector single antenna intercept type devcie.

Patrick, I think you have hit the nail on the head here. I would like to pick up any small and near Efield influences as well as any large and far Efield influences. The ability to distinguish them is not important to me. I would just like to be able to measure them. This is for fun and knowledge. I can't speak to any 'quack medical' detectors. What is the purpose of this is like saying, "What is the purpose of a TC?" <-- Sacrilege, I know...

Also, you are correct in chastising my use of "Amplified Noise". I think a better word choice maybe would have been "exaggerated noise".

As Efield's interact, charge is displaced. The larger and more intense the Efield, the more severely the charge is displaced. If the charge is displaced on one side of a Capacitor it will generate a voltage dip or rise across the capacitor. This small change should be able to be measured in the form of current through a small value resistor on the other side of a capacitor. Since I am producing a larger Efield, I am assuming I am going to get a Larger voltage across my cap/resistor.
Is any of the above not correct?

I am attaching a jpg of EXACTLY what I would like to do.

Additional thoughts are to replace the resistor with an inductor or put the resistor on the HV side. As you can see the Electroscope is merely verifying that I haven't bled down the voltage on my system. As stated earlier, charging will currently be accomplished by a computer monitor attached through a diode string. Crude, but simple.

Once again, I would like to reiterate my appreciation for the participation I am getting in this thread.
-NoSky

Antonio

Wed Oct 20 2010, 12:58AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Your meter connected in this way will only measure the leakage through corona at the antenna. With the capacitor charged and no corona the measurement will be zero.

Steve Conner

Wed Oct 20 2010, 12:54PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

E-fields don't "interact" with each other unless they're in a nonlinear medium.

I work for a company that makes partial discharge (corona for you Americans :) ) detection equipment for the power industry. When you're working in a 400kV substation, you can shake a bunch of keys and our RF detector will register huge amounts of RF. In fact some of our field guys got in the habit of using that to test that the detector is working.

But this is because your keys are a nonlinear medium: they get charged by the huge E-field of the substation's overhead busbars, and tiny sparks jump between them as you jingle them around. The sparks are the non-linearity that converts the power frequency field to RF.

Another way that E-fields can detect "influences" is: If charged objects are moved around, this changes the capacitance between them. If the total quantity of charge is kept constant (say the objects are insulated) then the change in capacitance results in a change of voltage. This is how the condenser microphone works: the tiny movements of the diaphragm are converted to an electrical signal. In the same way, a sensitive electrometer can detect you walking around the room because of whatever static charge happens to be on you. It can also detect charged clouds, raindrops and all sorts of other fun things.

Antonio

Wed Oct 20 2010, 10:54PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Something that can be made with an antenna connected to a charged capacitor and something to measure the current through the capacitor is a movement detector. Something moving close to the antenna will change the distributed capacitance C of the antenna, that is charged to a voltage V, and a current V dC/dt will flow through the charged capacitor. The sensitivity of the device is proportional to V.

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