Hello Surprise,
Read your post about making a shaking device.
I am hoping to build an electromagnetic shaker to 'shake' (move forwards and backwards) small metallic objects using two electromagnets and leaving one on while the other is off, and then switching them.
Well, as you may know, out of all the many different metals that are around (a lot), magnets will only work with iron. (and iron-based)
This can be useful for separating out bits of ferrous metals from non-ferrous, but it might not be of any help to you.
Many small metal objects are not iron. Most are not. Depends where you look of course. A bunch of keys, keys are unlikely to be magnetic, but the key ring would almost certainly be magnetic.
******************
Unless you are referring to small objects that you know to be magnetic.
Even so, i think a motor driven shaker would be better for most purposes.
If it is necessary to shake these small objects, and for some reason it is impractical to shake the container, then i would consider a large motor driven permanent magnet on a shaft, with pole-pieces to the container.
You can get some very strong permanent magnets these days, and making something like that would be easy.
Best of luck with it, John