In the ancient world - Greece, China etc. people observed that when some materials are rubbed they become attractive to small particles. They also observed that in complete darkness sparks can be seen in dry hair and maybe they connected that with the lightening they saw in the sky, but probably not. The records we have from Greece - record that when amber is rubbed with various materials the result is the weak attraction of small light objects.
To make sense of the process a little modern knowledge and modern materials are more helpful than the history book.
A large number of trials with many different materials leads to the conclusion that just two types of 'charge' exist -
NegativeLike charges repel and unlike charges attract. We call them positive and negative because there are just two types. Modern convention makes the electron negative. The ions that remain when electrons are removed from atoms are positive.
Look carefully at the ball in the clip. The clear plastic ball is full of dry air and a handful of two types of small light beads. The white ones are expanded polystyrene and the blue ones are of another plastic. Bouncing and shaking the ball charges the balls and the bag.
The white balls become positive and cling to the wall which is slightly negative.
Rubbing a plastic pen on my tie charges the pen positive and it chases the positive balls away.
Notice that the white balls repel each other. They all have a negative charge.
The colored balls behave in the opposite way. They are repelled from the wall that the white ones cling to. They collect in the bottom of the bag clinging to white balls in approximately equal numbers.
The negatively charged colored balls are attracted to the positive white balls.
Rubbing a patch on the bag on my trousers removes the negative charge and replaces it with a patch of positive charge. The negative colored balls now cling to the positive patch.
The positive pen has the opposite effect. The negative balls are attracted to the positive pen.
The bag and balls can be used to investigate the charging of a conductor by induction.
Example 1
The white balls are repelled from a finger brought close to the bag.
The finger is a conductor. (Rubbing a charged patch on the bag with a hand removes most of the charges.) The finger is brought close to the bag. The negative bag repels electrons from the tip leaving it positive. The positive finger now repels the positive balls.
Example 2
Bringing a finger up close to colored balls clinging to a positive wall has the same effect.
The positive wall attracts electrons to the tip, which becomes negative. The negative finger repels the negative balls.
Caution!
If there are very many negative balls clinging to a positive wall the net charge in the region (wall+balls) may be negative.
In this case the finger becomes oppositely charged and the balls are attracted.
Electric charge is measured directly with a digital Coulomb meter but until relatively recently electroscopes of various designs were still used regularly in schools if not for serious work.
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A word to the wise Electroscopes are cheap and instructive. They still feature in examination questions from time to time. In particular the sequence of steps required to charge an electroscope by electrostatic induction should be known. |