Print For centuries, the electroscope was one of the most popular instruments used by scientists to study electricity. Abraham Bennet first described this version in An electroscope is a device used to detect an electric charge. For centuries, it was one of the most popular instruments used by scientists to study electricity. His device, dubbed the versorium, consisted of a lightweight needle balanced on a pivot. The presence of electricity in a nearby object caused the needle to move.
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Pith ball electroscope from the s, showing attraction to charged object How it works In , Stephen Gray used a simple hanging thread, which would be attracted to any nearby charged object. In order to test the presence of a charge on an object, the object is brought near to the uncharged pith ball.
If the object is charged, the ball will be attracted to it and move toward it. The attraction occurs because of induced polarization  of the atoms inside the pith ball.
The pith is a nonconductor , so the electrons in the ball are bound to atoms of the pith and are not free to leave the atoms and move about in the ball, but they can move a little within the atoms. See diagram at right. If, for example, a positively charged object B is brought near the pith ball A , the negative electrons blue minus signs in each atom yellow ovals will be attracted and move slightly toward the side of the atom nearer the object. The positively charged nuclei red plus signs will be repelled and will move slightly away.
Since the negative charges in the pith ball are now nearer the object than the positive charges C , their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force. The pith ball can be charged by touching it to a charged object, so some of the charges on the surface of the charged object move to the surface of the ball.
Then the ball can be used to distinguish the polarity of charge on other objects because it will be repelled by objects charged with the same polarity or sign it has, but attracted to charges of the opposite polarity. Often the electroscope will have a pair of suspended pith balls.
This allows one to tell at a glance whether the pith balls are charged. If one of the pith balls is touched to a charged object, charging it, the second one will be attracted and touch it, communicating some of the charge to the surface of the second ball.
Now both balls have the same polarity charge, so they repel each other. The distance between the balls will give a rough idea of the magnitude of the charge.
Uses of Gold Leaf Electroscopes
The gold leaf electroscope This is an instrument for detecting and measuring static electricity or voltage. A metal disc is connected to a narrow metal plate and a thin piece of gold leaf is fixed to the plate. The whole of this part of the electroscope is insulated from the body of the instrument. A glass front prevents air draughts but allows you to watch the behaviour of the leaf. When a charge is put on the disc at the top it spreads down to the plate and leaf. This means that both the leaf and plate will have the same charge.