Understanding the Impact of Lead Length on Capacitance Measurements

[Darren93]

Hi, I'm looking at an experiment at the moment whereby I'm using an LCR meter to measure capacitance. However the effect of leads seems to be quite profound. Connecting up 50cm leads in a parallel setup has a capacitance of about 3pF and in a circle setup about 26pF. What causes such a capacitance and why does varying the shape cause a change in this?

 

[phinds]

What is a "circular setup"? It SOUNDS like something where the wires would be farther apart than in a parallel setup, so if that's the case then you must have made an error in your measurements since the capacitance should be dramatically less if you move the leads away from each other.

 

[Darren93]

That's what I would of thought. I checked several times and that's the readings the meter indicated. By a circle I mean a single loop where we just pulled apart the leads to form a circle. I have no idea what's going on to cause such a change. Could it be that there is a larger inductance of the wires that the reader is mistaking as a capacitance? Or is there any theoretical rational for why a circular wire would have a higher capacitance?

 

[phinds]

Hm ... I don't get it either.

 

[Plant_Boy]

I am quite interested in how this experiment was set up.

It could be that as you close the loop, the magnetic fields interact better to store energy.

$C= \frac {A \varepsilon}{d}$

As the distance, d, gets smaller, the capacitance, C, gets larger.

 

[phinds]

As the distance, d, gets smaller, the capacitance, C, gets larger.

Yes, that's the point. He is getting the OPPOSITE result which neither of us find reasonable.

 

[Plant_Boy]

Oh, Are we about to go into Lorentz forces here...?

 

[Darren93]

As long as I know now there is no real theoretical explanation as to why that would happen I can blame the meter readings giving off false readings. I'm pretty sure it is due to the inductance being larger, and as the meter works of determining phase shift from input to output it could cause problems with the capacitance measurments.

 

[berkeman]

I'm guessing it's 50/60Hz noise coupling into the circular arrangement and confusing the meter. Capacitance measurements of small value capacitances should be made with as short a length of leads as possible, and the unconnected capacitance reading should be subtracted from the measured value. I have leads about 5cm long on my hand-held LCR meter, and its unconnected reading is 0.28pF.