Economical Letter-Scales
Take a watch or small clock spring, and fix it by the center on a stick. At the other end attach a small brass hook to hold letters, etc., as shown in the figure.
At the top of the hook fix horizontally a small band, running over a strip of cardboard, likewise hanging on the stick.
Now graduate the cardboard strip with real weights, or their exact equivalents, and after this any small articles may be weighed with sufficient accuracy. The spring, being of steel, always turns to its original position when the scale is empty.