An electrometer measures units of electric charge. The standard electrometer is two thin leaves of gold foil suspended from an electrode. When the electrode is charged, the leaves repel each other and move apart. Their separation is a direct measure of the charge on the electrode.
Another way to construct an electrometer is to suspend two aluminum or copper foil pieces on wire (tantalum or platinum is best), or (far less expensively) nylon monofilament.
Charged electrometers are discharged by ionizing radiation. The most common radiation measurement device, the dosimeter, is actually a ruggedized, calibrated electrometer.
