Historical background of electric charges

Two millennia ago the Greeks noticed that amber (fossilised tree resin), after being rubbed with animal fur, would attract small bits of leaves and dust; the amber was said to have become 'charged'. Centuries later people found that a glass rod rubbed with silk cloth also attracts scraps of paper -- so glass, too, can be charged. Rubbing a charged rubber rod against another charged rubber rod produces repulsion, and rubbing a charged glass rod against another charged glass rod also produces repulsion, but bringing a charged rubber rod near a charged glass rod produces attraction. From these three observations came the inference that there are exactly two distinct kinds of electric charge, and that like charges repel while unlike charges attract. In the 18th century Benjamin Franklin named one kind of charge 'positive' (+) and the other 'negative' (-); by his convention, the charge that develops on rubber and amber is negative, while the charge that develops on glass is positive. Following the work of J. J. Thomson and Ernest Rutherford in the late 19th and early 20th centuries, it became clear that ordinary matter is made of atoms that are themselves electrically neutral, built from negatively charged electrons, positively charged protons, and electrically neutral neutrons. When two different materials are rubbed together (for example, rubber against silk), friction transfers some charge -- specifically, loosely-boun …