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Electromagnet Trouble Answered

I'm in the development stages for my entry into the Launch It contest. Right now I'm just experimenting (or trying to) with scale electromagnets. I'm using solid core insulated copper wire from RadioShack, and a nearly fresh 9v battery. I tightly wound the wire around a finishing nail, which happened to already be magnetic, but am not getting any improvement in the magnetic field. It seems to be the same, weak size. Am I doing something wrong, or do I just need to step the size up? Thanks!


(Do some research) Field strength is proportional to current, the more power you can put through this, the more you will get out. A car battery will power a starter motor (@ more than 50A I believe) and move the car too. Look at fewer windings with thicker wire, and a heavy power supply? L

Field strength is proportional to both current and the number of turns.

B=mu0in where I is current and n is the number of turns per meter per length

It is, but impedence is also proportional to the number of turns, and current is inversely proportional to that (or similar - I haven't checked) L

Impedance only comes into play with a change in voltage, so unless he's rapidly switching his power supply we can ignore it. (This is where Las Vegas comes in and points out some obvious fact I overlooked ;-))

DC through a coil generates a magnetic field, which opposes current flow. More turns (of the same grade wire) will reduce the amount of current (at the same voltage). L

No, back emf is only a factor when the field is building, once it's steady the only resistance is the material resistance..

Mmmm, I'm not going to bother reaearching this, but I might do an experiment with some wire. L

. Just thought of something. If you have a large coil (which it sounds like you will), there is the potential (pun intended) for the collapsing field to produce lethal voltage/current on power off. For smaller coils (relays, etc), a clamping diode is used, but this may not work with larger coils. Be careful.

When you mention isulated copper wire, is it enamelized copper wire? I think its just a problem of wire selection, you cannot use wires that are insulated with thick materials (ie PVC, rubber). Thinly insulated as possible, enamelized copper wire works the best (as mentioned by nachomahma) If you dont want to buy, you can source it from broken electric toy motors or even broken analog clocks. Just be sure to sand off the enamel at the point where you're attaching the battery.. Lastly, you may want to insulated the nail itself, and wind the wire on the insulation (a layer of electrical tape would do) to improve strength.

Do they make (preferably cheap) "enamelized" copper wire that is of a bigger thickness, thick enough to use a car battery with?

Yes they do, thickness as you mentioned is measured in AWG (also simply known as gauge), pretty much the rating for the wire.The lower the AWG, the thicker the wire.. A 22awg wire for example has a diameter of 0.025 inches. As for use with car battery, 14-16awg, wire seem ok for me (anyone care enlight?)..
In terms of pricing, these could give you some ideas
enamel magnet wire

> As for use with car battery, 14-16awg, wire seem ok for me (anyone care enlight?). . Depends - on the gauge of the wire and how long it is. For RNO's purposes, 14-16 may work (IIRC, 14 is good for 15-20A). But a car battery can put out 300+A, so, in some cases (eg, few turns), it might be way too small. . . Note to RedNeckOreo: If you use a large battery, be sure to use a fuse. 12VDC circuit breakers can save a lot of money on blown fuses, but are rather expensive on the front-end. And keep in mind that car batteries tend to explode around sparks/flames/etc.

. Oh yeah. Use cables, connected to the battery with the right terminals, with a few feet of wire going to a connection block. Borrow a couple of starter cables (you'll have to change the connector on one of them) from two of those cars on blocks that every good Redneck has in the front yard. heehee This will keep those pesky sparks that occur when you accidentally connect the coil with the switch turned on from igniting the battery.

. Try using a battery with more ampacity or more volts. A 9v transistor battery isn't going to drive a larger coil very well. . If you know how much wire you used, you can look up Ohms/linear measure for your wire and determine the resistance of the coil. This will give you a better idea of the voltage/current you will need. . You didn't say what kind of insulation was on your coil wire, but varnished/enameled wire works best for coils (the PVC insulation puts the coils too far apart). . How many turns to your coil? As a rule of thumb, the more turns the better.

So, use a battery with more ampacity and volts? Now, what I'm about to say would blow this scale off, and go big right now, but..... How 'bout a car battery? I would need some heavier gauge wire, I'm sure. I have some around here somewhere. Would the PVC insulation present a problem then? I would wind the wire around a metal tent stake. I was hoping to get a magnetic field of with about a foot radius, if not more. It's really experimental, and so-so at this point, I don't know if I'll get it made into an Instructable in time. Ah well, thanks for your help, and thanks to all others who responded!

. If I remember my Electricity classes right, a coil/ferrous core magnet produces a field that is primarily axial, not radial. Ie, the field is more concentrated at the ends, not the sides. But it's been awhile and I may have been napping that day.

Hey, almost didn't recognize you..you look good in yellow.. :)

I agree with both Nacho and Tool. You'd get better results from 2 to 3 D-Cell batteries than a 9V battery. It's not the voltage so much, as the current that drives your magnet.

Wow, I didn't even think of that. Man, y'all are good....

. We've just made the same mistakes already. :) Keeping exploring, just try to keep from killing yourself.


10 years ago

Ohm's law (or something similar) relates to this. You want to push lots of current to make magnetic field through as many turns as possible, but the more turns you have the more voltage you need to push the current because the resistance rises, limiting the current. Also you have to: - not exceed the maximum current of your wire or it will burn up - not exceed the current of your battery or it will go dead fast What I'm trying to say is there's a sweet spot... for any particular wire and battery, an optimum number of turns and voltage and amps for maximum strength. Beyond that, there are magnetic properties of the nail. There IS a maximum magnetic strength of it, but I don't have clear enough knowledge of it to explain.

I tightly wound the wire around a finishing nail, which happen to already be magnetic

This could be another reason, follow the right-hand-rule, and be sure the magnetic field created by the current, is not in opposing polarity as the nail's north and south. Or they will end up cancelling each other.

Increase either the number of turns or the current to increase the strength of the field. I'd guess the internal resistance of the 9 volt is too high, try with 6 c cells in series.