It was discovered centuries ago that certain types of materials would mysteriously attract one another after being rubbed together. For example: after rubbing a piece of silk against a piece of glass, the silk and glass would tend to stick together. Indeed, there was an attractive force that could be demonstrated even when the two materials were separated: Glass and silk aren't the only materials known to behave like this. Anyone who has ever brushed up against a latex balloon only to nd that it tries to stick to them has experienced this same phenomenon. Paraf n wax and wool cloth are another pair of materials early.
