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
Step 2: Put it together
Put it together so it looks similar to mine in the pics, of course it it can be completely different. Play around with it so you get the largest angle possible from your magnets.
Step 3: Write measurements onto graph paper
to make the scale on your graph paper it is handy to know that the average thumbtack weighs 0.25 grams and find out how much your countries coins weigh. Luckily here in New Zealand the new set of coins have just come out, and they are of a handy size. the 10cent weighs 3.3g, the 20cent weighs 4g and the 50cent weighs5g. Using these known weights draw up your scale by putting the known weights into the tray on your scale, mark the interval on your graph paper and keep doing this with more weights. I used coins aswell as thumbtacks to check wether they weigh the same amount and to make sure that the scale will be correct. Once you have marked all the intervals on the graph paper your set of scales is finished. mine was very sensitive up to one gram and then the interval rapidly decreased until the magnets touched, just after 10g.