What observations make it possible to assert that there are electric charges of different signs?

Hang two small pieces of cork next to each other on silk threads. We will touch each of them with a piece of electrically charged amber, while some of the electric charge will drain into each of the pieces of the cork. The silk threads to which they are suspended no longer hang vertically, but deviate at an angle. The plugs are now further apart from each other than they were before the charge was received. The same will happen if both pieces of cork are touched by electrically charged pieces of glass. If, however, one piece of cork is touched with charged amber and the other with glass, both pieces will be attracted to each other. This was the difference that led Du Fay to the assumption of the existence of two types of electric charge. A generalization has arisen: electric charges of the same name are repelled, unlike – are attracted.

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