Melting a nail with high currents

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Discussion Overview

The discussion revolves around the challenges of melting a nail using high currents generated by a transformer. Participants explore issues related to resistance measurements, current output, and the effectiveness of different measurement techniques. The scope includes technical explanations and experimental observations.

Discussion Character

  • Technical explanation
  • Experimental/applied
  • Debate/contested

Main Points Raised

  • One participant reports melting a nail with a transformer setup but questions the accuracy of resistance measurements obtained with a multimeter.
  • Another participant suggests that the multimeter's calibration, lead resistance, and contact resistance might affect the resistance readings.
  • A participant provides dimensions of the nail and wire, calculating resistance based on the resistivity of iron, which they find reasonable.
  • Concerns are raised about the stability of multimeter readings when measuring resistance directly on the nail, with suggestions to improve contact using crocodile clips.
  • There is a recommendation to measure current at high levels and calculate resistance from voltage measurements instead, along with a mention of specialized equipment like "4-wire ohmmeters" for low resistance measurements.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the accuracy of resistance measurements and the factors affecting them. There is no consensus on the best method for measuring resistance in this context, indicating multiple competing views on the issue.

Contextual Notes

Limitations include potential inaccuracies in multimeter readings due to contact resistance and the need for calibration. The discussion does not resolve the discrepancies in resistance measurements or the effectiveness of different measurement techniques.

greypilgrim
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Hi.
I melted a nail using a transformer with 500 windings on the primary and 5 on the secondary and 220 V input voltage, but I can't make the numbers work out. I measured the output voltage to be around 2 V, which makes sense.

Trying to measure the resistance of the nail directly with a multimeter yields around 6 Ohms, which appears be way to high. With a different power supply I can get 10 A across the nail at around 1 V, which leads to 0.1 Ohms, but this would still only lead to 20 A with the transformer, which still seems way too low to make it melt.

What's wrong here? And why is the resistance measured with the multimeter so far off?
 
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greypilgrim said:
What's wrong here? And why is the resistance measured with the multimeter so far off?
Was the multimeter calibrated? Did it have lead resistance and contact resistance? Measure it by shorting the test probes, and check the zero calibration.

What is the length and diameter of the iron? nail ?
What is the length and diameter of the copper? wire, used for the secondary winding ?
 
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The diameter is 4 mm and the distance between the contacts of the secondary is 5.4 cm. Using the resisitivity of iron, I get ##4\cdot 10^{-4}\enspace\Omega## , which seems about right.
I also found a different isolation transformer with an input ammeter that showed 3 A at 200 V, so the output current should be in the hundreds A, which makes much more sense.

First I just pushed the probes of the multimeter against the nail, which gave very unstable values. Then I used crocodile clips. Could it be that most of the resistance is due to the contacts? Should I maybe file down the surface of the nail?
 
greypilgrim said:
Could it be that most of the resistance is due to the contacts?
Yes.

Most DMMs just aren't designed to resolve very low resistance. You would probably be better off using it to measure current, at a high current level, and then voltage and calculating the resistance.

Read about "4-wire ohmmeters" or "kelvin connections".
 

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