What is a platform balance and how is it used?

A platform balance is an instrument to weigh things.

From Answers.com (see link to the left):
The platform balance is a form of equal-arm balance in which two flat platforms are attached to the top side of the beam, one at each end. Such a balance has a rider, or weight, mounted on a bar that has a calibrated scale, is parallel to the beam, and connects the supports of the two platforms. This rider is moved along the bar, its edge marking decimal fractions of the unit weight.

See the Related Questions to the left for more information about how to weigh a sample on a balance.