You will no doubt have come across the gold-leaf electroscope on which a charge will distribute itself so that if the solid pin and the flexible leaf have the same charge the leaf rises. With a bigger charge, the leaf rises more. This is useful, but is difficult to calibrate and so the electroscope is mainly used qualitatively.
We can see that the leaf rises until the moment of its weight about the ‘hinge’ H (clockwise in Figure 1) is equal to the anticlockwise moment of the forces between the charges. In other words, we could in principle equate the quantifiable amount of charge with a force, and forces are a lot easier to measure. That is what the basic electrometer does.
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