The balance is the oldest kind of scale. Other scales require sophisticated spring mechanisms to work, but the balance works on much simpler principles. By comparing an unknown object to known weights, you can come up with a precise weight for any object.
Get a thin rectangular piece of wood. Balance it on a pencil or on your finger. Use another pencil to make a line at the spot where it balances exactly.
Put a nail through the line so that it goes straight through the beam. It should be long enough to extend at least an inch on either side. Put a dab of glue on either side of the nail to stop it from moving in the hole.
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Screw an eyebolt into the end of each side of the beam. Make sure that both eyebolts are screwed in all the way.
Get two small plastic pails with handles on them.
Cut two short pieces of string of the same length. Tie the pails onto the eyebolts. You now have your scale assembled.
Make a stand. Cut a groove in the tops of two boards. Mount them on another board so that they are parallel. Set the nail in the grooves so that the buckets hang free.
Use the scale. Put what you want to weigh in one side. Add weights to the other sides until the weights equalize.
You can use lengths of chain instead of string for a more durable scale.