A pith ball hangs vertically by a thread. A plastic rod is charged by rubbing with cloth. The charged rod is brought close to the pith ball without touching it. It is observed that the charged rod weakly attracts the pith ball. This may be surprising – and you are right to be surprised, for the pith ball carries no charge. For the time being we are going to put this observation to the back of our minds, and we shall defer an explanation to a later chapter. Until then it will remain a small but insistent little puzzle.
We now touch the pith ball with the charged plastic rod. Immediately, some of the magical property (i.e. some of the electric charge) of the rod is transferred to the pith ball, and we observe that thereafter the ball is strongly repelled from the rod. We conclude that two electric charges repel each other. Let us refer to the pith ball that we have just charged as Ball A.
Now let’s do exactly the same experiment with the glass rod that has been rubbed with silk. We bring the charged glass rod close to an uncharged Ball B. It initially attracts it weakly – but we’ll have to wait until Section 3.6 for an explanation of this unexpected behavior. However, as soon as we touch Ball B with the glass rod, some charge is transferred to the ball, and the rod thereafter repels it. So far, no obvious difference between the properties of the plastic and glass rods.
But... now bring the glass rod close to Ball A, and we see that Ball A is strongly attracted. And if we bring the plastic rod close to Ball B, it, too, is strongly attracted. Furthermore, Balls A and B attract each other.
We conclude that there are two kinds of electric charge, with exactly opposite properties. We arbitrarily call the kind of charge on the glass rod and on Ball B positive and the charge on the plastic rod and Ball A negative. We observe, then, that like charges (i.e. those of the same sign) repel each other, and unlike charges (i.e. those of opposite sign) attract each other.