This is just an idea I had a while back. Instead of using springs for a set of scales I thought permanent magnets might do the trick. Unfortunately I didnt think about the properties of magnets, the magnetic field is not uniform and therefore the scale won't go up in even intervals, instead it seems to be an exponential or inverse relationship. But i though I would still publish it since it is somewhat interesting and later on in this instructable I'll tell you how you could get around the exponential problem.
Step 1: Materials
1) Meccano pieces
2) two permanent magnets preferably with holes (easy to attach to meccano frame)
5)A set of coins (find out how much they weigh)
6) plastic lid
7) thin wire and string
8) a sheet of cardboard or plastic