Message Forums

Quote: Originally posted by Gongoozler on 01/2/2010.... A single core of wire will have a magnetic field, but with 2 or 3 core mains cable the current will be flowing both ways and so the magnetic fields will cancel.

---

---

Quote: Originally posted by Rune Caster on 04/2/2010

---

Quote: Originally posted by Gongoozler on 01/2/2010.... A single core of wire will have a magnetic field, but with 2 or 3 core mains cable the current will be flowing both ways and so the magnetic fields will cancel.

---

---

If this were true then solenoids as in a/c relays would not work and neither would induction heaters, the magnetic field is there and will attract ferrous metals but won't magnetise them (you'll need DC for that)

My experience of this was when I borrowed a reel of 8mm cable from my father to supply power for lighting in a temporary theatre, he wasn't best pleased when he found out I hadn't unwound the complete reel and all the time it was in use we were happily playing with the magnetic field it was creating in the centre of the drum

Post last edited on 04/02/2010 15:40:16

---

When you wrap a mains cable around a drum you create two solenoids (ignoring the earth cable, but who needs one of them) with roughly equal but opposite currents. There will be some difference between the current in the live and in the neutral cable so the cancelation won't be perfect, hence the residue field in the middle of the drum.

If you had split the cable and wound the the live around one drum and the neutral around another one, thus seperating the two solenoids,  you would have had much stronger magnetic fields to play with (but I suspect your dad would have been even less pleased with you).

[B.T.W. This type of cancellation is basically the same principle as used in balanced microphone cables to stop them picking up the local taxi radios.]