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Re: dielectric constant



Here's how we did it and how we do it now.

The problem is measuring Q stored in the capacitor, V is easy (use a
voltmeter)

old way) We discharged the capacitor through some expensive Keithly meter
that had a charge integrator on it. This never worked well as the amounts
of charge were awfully small and tested the sensitivity of a persnickety
meter.

new way) We built what amounts to a 1mA constant current source (at least
when driven into modest voltages). So we charge of the capacitor while
timing how long we are charging it. The obvious multiplication of I times t
gives us a measure of how much charge we put on the capacitor. Cheap,
reasonably good results. The down side is the faith you must have that our
lab tech person really did build a constant current source; i.e. we don't
directly measure the charge.

another way that we don't do) Discharge the capacitor through a ballistic
galvanometer to measure the charge. We don't do this as we have only one
ballistic galvanometer set-up; (this is what we do to measure magnetic
fields with a search coil, having calibrated the galvanometer with the
discharge of a known capacitance.

Joel Rauber

-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]On Behalf Of Michael Bowen
Sent: Wednesday, January 31, 2001 3:24 AM
To: PHYS-L@lists.nau.edu
Subject: Re: dielectric constant


What's a cheap and easy way to measure the DC (or zero-frequency, for
purists) capacitance to within 5% or so for a beginning class? Or, how
do you do it even if your method isn't cheap and easy?