The Electroscope.  It has been shown experimentally that an electric charge can be detected because it attracts light objects such as pith balls, bits of paper, etc.

Any device used for detecting electric charges is called an electroscope.  In its simplest form, an electroscope consists of a pith ball hanging on the end of a silk thread.  By touching it with a body of a known charge, you have an instrument that can detect charged bodies and that can indicate the type of charge (polarity).

To illustrate, if you touch the pith ball with a glass rod, which has been rubbed with silk, you charge the pith ball positively.  Any other charged body that is brought near the pith ball will repel it if the body is positive or attract it if the body is negative.  The force of repulsion or attraction indicates the strength of the field surrounding the charged bodies.

A better and more sensitive device is the leaf electroscope shown in figure 1-7. It is two thin sheets of metal foil (usually gold or aluminum) called leaves, supported by a wire or stem whose ends pass through a block of sealing wax or insulating material to a metal ball or cap.

The leaves are usually sealed in a glass container to prevent air currents and moisture from affecting the instrument. The sensitivity of the instrument depends on several factors, the main two being the thickness and the type of material the leaves are made of.

If the ball receives either a positive or a negative charge, it causes the leaves to spread apart.  The leaves spread because like charges repel.  When a charge of positive electricity is placed on the leaves, the spread of the leaves will increase when the ball is approached by a positively charged body.  On the other hand, a negatively charged body brought near the ball or cap will decrease the spread.

You can place a charge on the leaves by bringing a charged body near, but without making physical contact with, the ball.  This is charging by induction.

As soon as you remove the charged body, the electroscope is no longer charged unless you provided some means for it to gain or to lose some electrons while the charge was being induced.  You can do this by connecting a wire from the electroscope to some neutral conducting object, such as ground.

Then, if a charged body is brought near the electroscope, electrons can leave if the charge is negative or enter if the charge is positive.  If the wire is disconnected before the charged body is removed, the electroscope will remain charged oppositely to the charge that induced it.

This is charging by conduction because the electroscope comes into direct contact with the charged body.

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