Expanding the Range of an Electrometer?

[Blaze_409]

If i have and electrometer. How can i safely increase its range from the 100V to the 10kV range. The electrometer has ultra high resistance so whatever i added in the circuit would need that too. Any ideas?

 

[omoplata]

Measure its resistance, then connect a resistor that has 9 times that resistance in series?

 

[Blaze_409]

Measure its resistance, then connect a resistor that has 9 times that resistance in series?

The electrometer has a resitance of 10^14 ohms. Thats enought to qualify it as an infinite impedance electrometer. I can't imagine havgin 9 times that resitance although I am sure it could be done

 

[omoplata]

Oops, made a mistake. Did you say 100V to 10kV? Then it has to be 99 times the resistance because 10kV/100V = 100.

The theory is http://en.wikipedia.org/wiki/Potential_divider" .

 

[Antiphon]

A resistive voltage divider will ruin the input impedance of your electrometer turning it into a voltmeter. They are different instruments.

You need an electrostatic field divider. This is something you'd design by solving the Laplace equation but the principle is simple.

You place the electrode of your electrometer into a metal box with a hole in the top. By adjusting the geometry of the box and hole, you can get the electrometer field to be 1% of the field outside the box. This is conceptually different from a voltage divider because the latter requires current to flow where the shielding solution does not.

Good luck.

 

[omoplata]

I thought electrometers are highly sensitive voltmeters (http://en.wikipedia.org/wiki/Electrometer#Modern_electrometers").

 

[Antiphon]

I thought electrometers are highly sensitive voltmeters (http://en.wikipedia.org/wiki/Electrometer#Modern_electrometers").

Technically yes. But no.

Electrometers are highly sensitive electrostatic voltmeters. What we call voltmeters are actually current meters that work according to the resistive voltage divider principle.

If you take an electrometer and connect it to a resistive voltage divider, it will become a voltmeter and read zero volts because no current is flowing through the air.

An electrometer on the other hand is a true potential meter or Voltmeter. It would be able to register the existence of a fixed charge and actually give you the true voltage- something an ordinary voltmeter can't do.

 

[omoplata]

Thanks for clarifying. I will write down my understanding of the matter below. Please correct me if I'm wrong.

The terms in bold are terms I defined because I couldn't find any standard terms for those instruments.

Electrometers are highly sensitive electrostatic voltmeters.

So a modern electrometer (like "ftp://ftp.pasco.com/Support/Documents/English/ES/ES-9078/012-07124b.pdf"[/URL] for example) is a highly sensitive voltmeter.

[quote="Antiphon, post: 3318995"]What we call voltmeters are actually current meters that work according to the resistive voltage divider principle.[/QUOTE]

Then a [B]modern electrometer[/B] a highly sensitive current meter that works according to the resistive voltage divider principle. Therefore it can be used in a voltage divider circuit, since a current flows through it.

What I don't get is how it can be electrostatic and have a current flow through it at the same time. So I assume it is [I]called[/I] electrostatic because the current going through it is very small. But the actual principle of operation of the instrument requires a current to flow through it, so it still can be used in a voltage divider circuit.

An [B]ideal electrometer[/B] on the other hand does not have any current flowing through it. It measures the potential difference by measuring the electrostatic force. For example [PLAIN]http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Quandrant_Electrometer/Quadrant_Electrometer.html" is an ideal electrometer. I guess they are not used anymore because modern electrometers are much more accurate.

An electrometer on the other hand is a true potential meter or Voltmeter. It would be able to register the existence of a fixed charge and actually give you the true voltage- something an ordinary voltmeter can't do.

What you refer to in the above quote is an ideal electrometer. This cannot be used in a voltage divider circuit.

What I don't understand, however, is the following quote.

If you take an electrometer and connect it to a resistive voltage divider, it will become a voltmeter and read zero volts because no current is flowing through the air.

Are you referring to an ideal electrometer here? If no current ever flows through it, then it's input impedance is infinite and whatever resistance that is connected to it in series as part of the voltage divider circuit can be ignored. So it will measure the voltage applied to the input of the voltage divider.

On the other hand, if you are referring to a modern electrometer, it can have a current that flows through it (very small maybe, but still a non-zero current) and therefore will have a finite input impedance. So it will measure a voltage less than that applied to the input of the voltage divider.

So it seems to me that if Blaze_409 has a modern electrometer then he/she can use it in a voltage divider circuit. But if he/she has a ideal electrometer then using it in a voltage divider circuit will have no effect.

Please correct me if I'm wrong.

 

[Blaze_409]

Thanks for clarifying. I will write down my understanding of the matter below. Please correct me if I'm wrong.

The terms in bold are terms I defined because I couldn't find any standard terms for those instruments.



So a modern electrometer (like "ftp://ftp.pasco.com/Support/Documents/English/ES/ES-9078/012-07124b.pdf"[/URL] for example) is a highly sensitive voltmeter.



Then a [B]modern electrometer[/B] a highly sensitive current meter that works according to the resistive voltage divider principle. Therefore it can be used in a voltage divider circuit, since a current flows through it.

What I don't get is how it can be electrostatic and have a current flow through it at the same time. So I assume it is [I]called[/I] electrostatic because the current going through it is very small. But the actual principle of operation of the instrument requires a current to flow through it, so it still can be used in a voltage divider circuit.

An [B]ideal electrometer[/B] on the other hand does not have any current flowing through it. It measures the potential difference by measuring the electrostatic force. For example [PLAIN]http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Quandrant_Electrometer/Quadrant_Electrometer.html" is an ideal electrometer. I guess they are not used anymore because modern electrometers are much more accurate.



What you refer to in the above quote is an ideal electrometer. This cannot be used in a voltage divider circuit.

What I don't understand, however, is the following quote.

Are you referring to an ideal electrometer here? If no current ever flows through it, then it's input impedance is infinite and whatever resistance that is connected to it in series as part of the voltage divider circuit can be ignored. So it will measure the voltage applied to the input of the voltage divider.

On the other hand, if you are referring to a modern electrometer, it can have a current that flows through it (very small maybe, but still a non-zero current) and therefore will have a finite input impedance. So it will measure a voltage less than that applied to the input of the voltage divider.

So it seems to me that if Blaze_409 has a modern electrometer then he/she can use it in a voltage divider circuit. But if he/she has a ideal electrometer then using it in a voltage divider circuit will have no effect.

Please correct me if I'm wrong.

There is only 1 type of elecctrometer. (theres no such thing a a modern/ideal electrometer). Its just an electrometer. The concept of an electrometer is a that it has an infinite input impedance.( anywhere around 10^14 or greater ohms) so it does not draw any current from the system you connect it to. will not load the system.


A voltage divider circuit as stated above draws current. You can't afford to lose current when your trying to measure wit electrostatics. That kills the entire thing.




Also using an "ideal electrometer" in a voltage divider would have an effect. The circuit is no longer infinite impedance and therefore loads down the system too much

 

[omoplata]

There is only 1 type of elecctrometer.

I showed links to at least two types. One draws a small current. One doesn't.

(theres no such thing a a modern/ideal electrometer). Its just an electrometer.

Modern and ideal were terms I temporarily defined to make my point clear. One drew a small current. The other didn't.

The concept of an electrometer is a that it has an infinite input impedance.( anywhere around 10^14 or greater ohms) so it does not draw any current from the system you connect it to. will not load the system.

10^14 is finite.

Also using an "ideal electrometer" in a voltage divider would have an effect. The circuit is no longer infinite impedance and therefore loads down the system too much

So what's the input impedance of the attached circuit diagram?

 

[Blaze_409]

I showed links to at least two types. One draws a small current. One doesn't.

Modern and ideal were terms I temporarily defined to make my point clear. One drew a small current. The other didn't.

10^14 is finite.


So what's the input impedance of the attached circuit diagram?

why are you being so literal. 10^14ohms qualifies as almost infinite impedance.

 

[Blaze_409]

Are you referring to an ideal electrometer here? If no current ever flows through it, then it's input impedance is infinite and whatever resistance that is connected to it in series as part of the voltage divider circuit can be ignored. So it will measure the voltage applied to the input of the voltage divider.

On the other hand, if you are referring to a modern electrometer, it can have a current that flows through it (very small maybe, but still a non-zero current) and therefore will have a finite input impedance. So it will measure a voltage less than that applied to the input of the voltage divider.

So it seems to me that if Blaze_409 has a modern electrometer then he/she can use it in a voltage divider circuit. But if he/she has a ideal electrometer then using it in a voltage divider circuit will have no effect.

Please correct me if I'm wrong.

1. Electrometers etc never draw voltage.( they can only draw current).
2.weve been over the voltage divder circuit and that has resistors that put a load on the enerator. that makes the electrometer useless.


Voltage dividers are menat to be used in things that have current.