A digital balance has uses in almost every industry, from chemistry, to pharmaceuticals, to composite materials. Digital balances have plummeted in cost in recent years, and 10 or 5 milligram units can be had on ebay for less than 20 dollars. Scales with milligram or less resolution, however, are still very expensive, with large units into the thousands of dollars.
This procedure describes a simple balance capable of resolution well below 1 milligram. in short, a photo diode adjusts the current through a galvanometer coil in order to provide a constant force to lift the sample to be weighed. galvanometer hardware is an excellent match, since the coil is already wound, and the sapphire bearings are extremely low friction.
DIY Digital Ballance: [Link]