According to Concord Consortium, pith-ball, gold-leaf, and needle electroscopes measure electric charges using Coulomb electrostatic forces. Pith-ball electroscopes use attraction as a measure of charge, while gold-leaf and needle electroscopes use repulsion. These early instruments helped scientist understand and measure electric charges and polarity. Today they serve primarily as demonstration instruments.
According to Physics Tutorials, the pith-ball electroscope takes its name from wood pith, the light spongy interior of a tree. The dried pith ball, uncharged and suspended on a thread, moves toward a charged object. The greater the charge the more the pith ball moves. A charged pith ball moves toward or away from a charged object based the object’s polarity. Modern pith-ball electroscopes use a plastic ball, and sometimes use two to demonstrate repulsion.
Physics Tutorials explains that gold-leaf electroscopes have a metal rod with two thin gold strips at the end that can move freely. When a charge is applied to the rod the gold leaves swing apart due to repulsive forces. To improve sensitivity, the assembly uses a glass housing that eliminates drafts. Gold-leaf electroscopes have much better sensitivity, and they remained precision instruments until the mid 19th Century.
Needle electroscopes use the same repulsion principle, but instead of gold leaves, a lightweight needle pivots on the charging rod. Any charge on the rod causes the needle to swing away. These instruments also use glass enclosures because drafts can affect the pivoting needle.