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Electrical Engineering/Electromagnet may be going wrong somehow


Hello again Cleggsan,

I was wondering if you might be able to comment on the make up of an electromagnet I have made that does not seem to be delivering the magnetic field strength I thought it would. I have seen various videos and web pages outlining the making of an electromagnet where they are able to pick up paperclips. My electromagnet is barely able to pick up a few staples and I was wondering if you could tell me if I have gone wrong somewhere.
I am using two 9V batteries in parallel and about 4 meters of insulated stranded copper wire with 12 x 0.18mm diameter copper strands (total diameter 2.16mm). My core is made of metal washers with a diameter of 3cm, stacked to a height of about 8cm. I estimate there are around 40 turns. I have checked the washers are made of magnetic material by putting a permanent magnet to the washer and verifying it is attracted.
I have used a lot of electrical gum tape to hold the whole thing together - around the washers and then also around the wire.
Does a magnetic field that only attracts three staples seem right here? Could I have done something wrong that would reduce the magnetic field in some way? Once I turn off the magnet the staples seem to have a higher degree of magnetism than when they were attracted to my electromagnet. I put some metal scissors to the staples and about 10 of them were attracted to the scissors - far more than were attracted to the electromagnet. I am stumped by this.
Any thoughts would be much appreciated!

Thank you and best regards,

I'd say it's the core you are using.  You'd be better off with a solid iron slug.  The washers are not very magnetic when stacked compared with solid core.

And, the coil must be wound tightly around the core.  Small spaces make for big reduction in magnetic flux getting through and around.

The above is nice one to look at.  Than look at the document and the video listed under the coil on page one of the link above.  

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