# How to increase an electromagnet's magnetic field size?

I made this electromagnet out of a microwave oven transformer by taking out the secondary coil. It very strong but it can only attract the metals if I bring it really close. How do increase the distance at which it can attract the metals?

• I've taken two very strong permanent magnet speakers and put them together back to back , with a screw further attaching them. Then I wrapped a 12' extension cord around them. Plug it in, and it works as an excellent line conditioner/surge arrester too. Mar 25, 2016 at 15:36
• @abubakr, I know you are new here but don't you think a photo would help anyone interested in helping you? On this site you thank afterwards by accepting and up-voting answers. Mar 25, 2016 at 19:02
• The force of an electromagnet is proportional to the current flowing through it and the permeability of the core, so unless you change those, I don´t think you can. Mar 25, 2016 at 19:02
• I know that much already, but like i specifically said in the question, Its very strong but it can only attract the metals if i bring it really close. How do i INCREASE THE DISTANCE at which it can attract the metals. Mar 25, 2016 at 19:07
• Well, maybe extend the core with another piece of suitable metal (iron)? What exactly are you trying to make here? Mar 25, 2016 at 19:32

You need to think about the path of the magnetic field. The field is not something that flows like an electric current is electrons flowing through a conductor, but it is convenient to picture it that way. The magnetic field flows in a closed path through the center of the coil, out one end and and around the outside of the coil the the other end. The field flows most easily through iron, but if there is no iron, in the path, it will flow through the air to reach the other end of the coil. It will take the path that has the minimum distance through air. The shape of the microwave oven transformer (MOT) core tends to keep the magnetic flux close to the core. If the core was a straight bar, the flux would tend to reach out further. Also the force on another piece of iron increases as the iron moves closer to closing the air gap. Here is a diagram that shows the magnetic flux in red.

If you are energizing the magnet with AC, it might be a good idea to try DC. You might consider winding a new secondary winding with larger wire and connecting the secondary to a high-current rectifier to provide low voltage DC the way MOTs are used to make welders and jump-starting power supplies. The DC supply could then supply high current to a bar-shaped electromagnet. With DC, you wouldn't need laminated transformer steel, but would need a thick bar, at least as thick as the MOT core. It might be more convenient to build it up from smaller flat bars.

You should probably study various descriptions of MOT electromagnet and other projects that you can find on the internet. Details are different among the various projects, so you might find a different way of doing things that is more suitable for the outcome that you want. Also look for material to study about electromagnets in general.

• is their a way to calculate its size Mar 25, 2016 at 21:34
• The size of the magnetic field? I believe it theoretically extends an infinite distance from the magnet, but the strength is reduced dramatically as the distance increases. The strength at any distance and direction can be calculated. I believe that you would first calculate the strength at the surface of the iron as determined by the size of the iron, the permeability of the iron, the number of turns of wire, the current and the geometry of the electromagnet. Then you would calculate the strength of the field at point at a given distance and direction. The details can be found in textbooks.
– user80875
Mar 25, 2016 at 22:20

The distance of the magnetic field reaching out in space depends greatly on the length of the magnet and its strength. If we take for example the magnetic field of the earth, it reaches out thousands of km despite of the relatively weak field strength, due to the enormous size of earth. You could try with a straight long ferromagnetic core to increase the distance. Also perpendicular placed permanent magnets similar the Halbach arrays could work.

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