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Physics/insulated wire


thank you, please tell me: so in an experiment i set up where i had a multimeter insulated up on a PVC pipe with one probe on a 25kV HV DC (-) terminal and the other probe terminating with either a needle or a one meter length of insulated wire attached; to measure the current going out of the two different terminals into the air. the results were <3 micro amps for the needle and >10 micro amps for the insulated wire. the wire had a rounded end with a insulative material. why does the wire have more current going through it then the sharp needle? is it the surface area that compensates for the lack of any sharp points on the wire terminal. i noticed that the wire length was crucial to the current flow, as a 0.5m wire was only measuring 2 microamps.
it is not that the wire was closer to ground as i moved it around with no change unless i did have it closer to a large object. the wire is smooth and insulated there are no bits sticking out that could cause a discharge yet it has 3 times the current going out through it. i noticed that when the wire was shorter than attaching a needle to the end of it increased the current significantly but when the length of wire was ~1m the needle made no difference. is it that the wire is better at ionising the air or is it that the wire is acting as a capacitor and has a capacitance that gives this effect?
what do you think,

Hello gene,

When you ask "is it the surface area that compensates for the lack of any sharp points on the wire terminal." Are you referring to the insulated surface of the 1 m of wire? Assuming so, I might agree if the insulation is not suitable for hi voltage applications. If the insulation is worthy of the task of insulating a wire carrying -25kV HV DC, then surface area should not explain it.

I wonder about your multimeter. Is it rated for high voltage work? If not, it might be doing something weird because it is trying to work in a situation it was not designed for. The significant change in the reading when you shortened the wire makes me suspect that the voltage is not pure DC. There might be significant ripple on it and changing the length might be affecting a resonance. Do you have access to an oscilloscope? Bringing a scope probe close to the output (don't touch the output with the probe unless it is intended for high voltage) would probably show ripple if there is any.

Capacitance would not have an affect, unless the -25kV HV DC is carrying significant AC ripple with it. I should add a caution: these are the opinions of a Physicist/Electrical Engineer who has not had much experience working with high voltage.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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