# Why are you supposed to wind an electromagnet only one way?

As I have been researching winding electromagnets, one of the suggestions that came up was to only wind the magnet in one direction - not just as in always spinning the rod one direction, but as in winding down the rod, then to wind the second coil by running the wire back up the coil parallel to the rod and then taping over and winding back down in the same direction (as described here). I'm reasonably sure that this is not just my misreading since my physics teacher also remembered a similar instruction for building strong electromagnets.

Why is this? I can't think of any explanation since it seems to me that as long as the rod is not rotated in the opposite direction while winding, the electricity will be flowing in approximately the same direction around the rod, therefore generating aligned magnetic fields.

• From reading the linked article, the directions are to stick to CW or CCW and that's it. There should be no problem with winding up and down, back and fourth. (none of this from top to bottom only business) Jun 6 '17 at 19:32
• Hint: Research "Ampere's right hand rule". Note that if the current-carrying wire is wrapped (oriented) so that the coiled wire's magnetic B fields are in opposition (some coils are wound CW, other coils are wound CCW), the B fields will cancel each other out, and will weaken or completely eliminate the electromagnetic effect. Jun 6 '17 at 23:39
• @JimFischer That is not what I am talking about - I tried to specify in the question that this is not about winding CW and CCW on the same rod. All the wraps are either CW or CCW. The question is asking about the direction up or down the rod wrapping is proceeding.
– dpdt
Jun 7 '17 at 0:24

There is one reason to fly-back and wind in the same direction, that is, the subsequent winding knits neatly into the grooves between the turns on the previous winding. It does not seem much but it ends up being a tighter wind rather than winding in the other direction.

Ultimately, if the central core is not too small a diameter, you can fit more turns into the same volume.

I have even seen this done winding multiple times then joining the ends over top after all the layers are complete.

The classic way to create non-inductive wirewound resistor is take twisted pair wire, and wrap around a spool.

Thus if you wind both ways, you cancel some/all of the inductance.

• Not sure that has anything to do with the question... but interesting add anyway :) Jun 6 '17 at 19:53
• Agreed. Cool info, but I'm not talking anything about non-inductive resistors, so reluctantly downvoted.
– dpdt
Jun 7 '17 at 0:25
• I'm just illustrating the need to PAY ATTENTION to your direction of winding. Once you run that wire back up, parallel to axis, be sure to continue winding in the original direction. Jun 7 '17 at 12:17