Re: DIY electromagnet under £100: Disappointing results

Hi everyone,

> > Just an update of what I've done. > > My 125m long coil of enameled 0.8mm copper wire had a resistance of > ~4.4 Ohms. With a power supply of 12V & 34A, this meant I needed a > length of about 10m to get the 0.35 ohms of resistance to maximise > current flowing through the coil. > > After cutting a 10m length of wire, stripping the ends and measuring > resistance, it had a resistance of 1.1 Ohms. I guess the higher than > expected value was due to contact resistance, which appears to be > quite significant (and hard to estimate). Out of curiosity will the > contact resistance drop once minor arcing or melting of the low > surface area contact points occurs due to high current densities? > > I wrapped the wire around a 5cm long 10mm steel hex bolt. I shall > worry about more complicated magnetic forms to have the test piece > "complete the circuit" later. > > I switched on the power supply and wham! The bolt stuck firmly to > another bolt nearby. I was able to lift it off the table with its own > magnetic pull strength! :) > > I burnt my finger holding the 12V lead to the bare end of the copper > wire :( but apart from that am a happy person now :). The system > generates a lot of heat and just switching it on for a few seconds > made it warm to touch. > > Thanks again! > > Jessie xx

So Jeese, is your supply able to deliver 35 amps? Someone pointed out that a 500 watt supply wouldn't be able to.

125 meters (410ft) of wire should have about 4.1 ohms. 4.4 ohms may be meter lead error. 10 meters (32.8ft) should measure 0.328 ohm, 1.1 ohm is way off, check the connections of your meter leads to the meter. Glad to hear your progress. Mike
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