The item in photo is a coil of 20-gauge copper wire (with enamel or varnish insulation), is 12 inches in diameter, weighs close to 5 pounds and has a resistance of about 25 ohms. The coil is wrapped with electrical tape. Appears to have been homemade. Found it in a box of electrical odds & ends which I rescued, to keep some potentially good (?) stuff from going to the landfill.
A couple of thoughts on the possible intended use -
- Sense coil for a home-brew metal detector.
- Degauss coil for use with CRT monitors, TVs., etc..
Bob M.
I would have gone with a degaussing coil ! It looks a bit like the ones we had 30 odd years ago. Having said that when I was at school I recall some apparatus with a similar coil used for demonstrating the principles of magnetism.
It does look a good bit like commercially available degaussing coils. With CRT monitors being well on the way toward the same end as the dinosaurs, it's probably not worth even experimenting to see if it works for that task. Also, if my measurement of 25 ohms is right, it would pull about 4.5 amps when hooked up to 115 vac outlet. Guess that would make me a little nervous. Possibly a very brief activation is all it takes though. Anyway, thanks much. Maybe the best use for this item is taking it to the metal recycler!
--- For the current:
#20 AWG weighs about 3lb. per 1000', so 5lb. would be:
3lb 5lb ------- = ------- = 1667 feet long 1000' x'A 12" diameter loop has a circumference of
C = pi * D ~ 37.7"
So the loop has:
1667' * 12" N = ------------ ~ 530 turns 37.7"on it.
Now, go to:
and fill in the boxes:
N = 530 turns R = 0.152 meter (radius of circle) a = 0.000081 meter (radius of wire) µr = 1 (relative permeability of air)
Click on "calculate" and 0.3216537332062735 will come up, so your coil has an inductance of about 322 millihenries.
At 60 Hz it'll have a reactance of:
Xl = 2 pi f L = 6.28 * 60Hz * 0.322H ~ 121.3 ohms
and, since it has a resistance of 25 ohms, it'll have an impedance of:
Z = sqrt (R² + Zl²) = sqrt (625 + 14714) ~ 124 ohms.
The current through it then, will be:
E 120V I = --- = ------ = 0.967 amperes. Z 124R
Since the reactive part of the impedance is lossless, the slick part is that the coil will only be dissipating power in its resistance, and with 967 milliamperes going through its 25 ohms, that comes out to:
P = I²R = 0.967A² * 25R ~ 23 watts.
So, while it'll warm up somewhat, I'd bet that it wouldn't get can't-keep-my-finger-on-it hot no matter how long you left it connected to the mains.
---
--- Seems like a shame... ;)
JF
Wow, another example of what an amazing network of expertise one can tap into via the net, specifically, through Google groups! Thanks much, John.
Those old degaussing coils have found new life as tool demagnetizers. If it doesn't explode the first time you plug it in, hang on to it.
Tom
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