How to measure a capacitance of an isolated object?

1. Nov 12, 2012

Artlav

How can you measure a capacitance of an isolated object, like a conductive sphere or a coil?

Can it be done using a typical RLC meter?
I've tried touching only one end of the probe to the object in question, and that seems to give sporadic readings, but within the order of magnitude of the theoretical expectation.

How to do it reliably, if there is a way?

2. Nov 12, 2012

Staff: Mentor

By definition, capacitance is between two objects or surfaces. You need to measure the capacitance of your "isolated" object with respect to something.

3. Nov 15, 2012

cabraham

I believe that the capacitance of a single object is measured wrt a sphere of large radius approaching infinite. E.g. the capacitance of the earth is around 710 uf, IIRC from memory. The large sphere enclosing the object is 1 electrode, with the object being the other. Taking the limit as the radius of the sphere approaches infinite gives the capacitance of the object. I believe Halliday & Resnick physics texts have illustrative examples of this.

Did I help?

Claude

4. Nov 15, 2012

tiny-tim

hi claude!

but how do you measure it?

5. Nov 15, 2012

cabraham

I'm not sure as to how it is measured. Capacitance of a single object is valid theoretically, but I'm not sure about it being a measurable quantity. I would say a good approximation is to enclose said object inside a sphere whose radius is large compared with the object dimensions, then measure capacitance between object & sphere. That should get you in the ball park. Of course as the sphere radius increases, the measurement gets more accurate, eventually converging. Maybe tonight I'll look this subject up. I have several good reference texts. Off the cuff, that would be my approach. Comments welcome.

Claude